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The Spaceguard Surve 



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Report of the 

NASA International 
Near-Earth-Obj ect 
Detection Workshop 

David Morrison, Chair 



January 25, 1992 



Cover: An image of 951 Gaspra, the only asteroid yet imaged by a spacecraft, overlays 
a diagram of the orbits of the inner planets and 100 of the largest known near-Earth 
asteroids. 



January 25, 1992 

The Spaceguard Survey: 

Report of the 

NASA International 

Near-Earth-Object 

Detection Workshop 

edited by 
David Morrison 

WORKSHOP MEMBERS 

Richard Binzel (Massachusetts Institute of Technology, USA) 

Edward Bowell (Lowell Observatory, USA) 

Clark Chapman (Planetary Science Institute, USA) 

Louis Friedman (The Planetary Society, USA) 

Tom Gehrels (University of Arizona, USA) 

Eleanor Helin (Caltech/NASA Jet Propulsion Laboratory, USA) 

Brian Marsden (Harvard-Smithsonian Center for Astrophysics, USA) 

Alain Maury (Observatoire de la Cote d'Azur, France) 

Thomas Morgan (NASA Headquarters, USA) 

David Morrison (NASA Ames Research Center, USA) 

Karri Muinonen (University of Helsinki, Finland) 

Steven Ostro (Caltech/NASA Jet Propulsion Laboratory, USA) 

John Pike (Federation of American Scientists, USA) 

Jurgen Rahe (NASA Headquarters, USA) 

R. Rajamohan (Indian Institute of Astrophysics, India) 

John Rather (NASA Headquarters, USA) 

Kenneth Russell (Anglo-Australian Observatory, Australia) 

Eugene Shoemaker (U.S. Geological Survey, USA) 

Andrej Sokolsky (Institute for Theoretical Astronomy, Russia) 

Duncan Steel (Anglo-Australian Observatory, Australia) 

David Tholen (University of Hawaii, USA) 

Joseph Veverka (Cornell University, USA) 

Faith Vilas (NASA Johnson Space Center, USA) 

Donald Yeomans (Caltech/NASA Jet Propulsion Laboratory, USA) 

OUTSIDE REVIEWERS 

Alan Harris (Caltech/NASA Jet Propulsion Laboratory) 
Paul Weissman (Caltech/NASA Jet Propulsion Laboratory) 
George Wetherill (Carnegie Institute of Washington) 
Kevin Zahnle (NASA Ames Research Center) 



ill 

PRECEDING PAGE BLANK NOT FILMED 



This report was prepared at the Jet Propulsion Laboratory/California Institute of Technology, 
Pasadena, California, for NASA's Office of Space Science and Applications, Solar System 
Exploration Division, Planetary Astronomy Program. 



Editorial Support: Anita M. Sohus, Flight Projects Office 
Layout and Illustration: Robin C. Dumas, Documentation Section 



January 25, 1992 



IV 



EXECUTIVE SUMMARY 

Report of the NASA International Near-Earth Object Detection 

Workshop 

Background. Impacts by Earth-approaching asteroids and comets pose a significant hazard 
to life andproperty. Although the annual probability of the Earth being struck by a large asteroid 
or comet is extremely small, the consequences of such a collision are so catastrophic that it is 
prudent to assess the nature of the threat and to prepare to deal with it. The first step in any 
program for the prevention or mitigation of impact catastrophes must involve a comprehensive 
search for Earth-crossing asteroids and comets and a detailed analysis of their orbits. At the 
request of the U.S. Congress, NASA has carried out a preliminary study to define a program for 
dramatically increasing the detection rate of Earth-crossing objects, as documented in this 
Workshop Report. 

Impact Hazard. The greatest risk from cosmic impacts is associated with objects large enough 
to disturb the Earth's climate on a global scale by injecting large quantities of dust into the 
stratosphere. Such an event would depress temperatures around the globe, leading to massive 
loss of food crops and possible breakdown of society. Such global catastrophes are qualitatively 
different from other more common hazards that we face (excepting nuclear war), because of their 
potential effect on the entire planet and its population. The possibility of such a global 
catastrophe is beyond question, but determining the threshold impactor size to trigger such an 
event is more difficult. Various studies have suggested that the minimum mass impacting body 
to produce such global consequences is several tens of billions of tons, resulting in a groundburst 
explosion with energy approaching a million megatons of TNT. The corresponding threshold 
diameter for Earth-crossing asteroids or comets is between 1 and 2 km. Smaller objects (down 
to tens of meters diameter) can cause severe local damage but pose no global threat. 

Search Strategy. Current technology permits us to discover and track nearly all asteroids or 
short-period comets larger than 1 km diameter that are potential Earth-impactors. These objects 
are readily detected with moderate-size ground-based telescopes. Most of what we now know 
about the population of Earth-crossing asteroids (ECAs) has been derived over the past two 
decades from studies carried out by a few dedicated observing teams using small ground-based 
telescopes. Currently, several new ECAs are discovered each month. At this rate, however, it will 
require several centuries to approach a complete survey, even for the larger objects. What is 
required to assess the population of ECAs and identify any large objects that could impact the 
Earth is a systematic survey that effectively monitors a large volume of space around our planet 
and detects these objects as their orbits repeatedly carry them through this volume of space. In 
addition, the survey should deal with the long-period comets, which are thought to constitute 
about 5 to 10 percent of the flux of Earth impactors. Long-period comets do not regularly enter 
near-Earth space; however, most Earth-impacting long-period comets could be detected with 
advance warning several months before impact, using the same telescopes used for the ECA 
survey. Finally, it is desirable to discover as many of the smaller potential impactors as possible. 

Lead Time. No object now known has an orbit that will lead to a collision with our planet during 
the next few centuries, and the vast majority of the newly discovered asteroids and comets will 
also be found to pose no near-term danger. Even if an ECA has an orbit that might lead to an 
impact, it will typically make hundreds of moderately near passes before there is any danger, 
providing ample time for response. However, the lead time will be much less for a comet 
approaching the Earth on a long-period orbit. 

Spaceguard Survey Network. The survey outlined in this report involves a coordinated 
international network of specialized ground-based telescopes for discovery, confirmation, and 
follow-up observations. Observations are required from both the northern and southern 
hemispheres, monitoring about 6,000 square degrees of sky per month. In order to provide 
reliable detection of objects as small as 1 km diameter within a suitably large volume of space, 



the telescopes should reach astronomical magnitude 22. The telescopes that are suitable to this 
survey have apertures of 2 to 3 meters, moderately wide fields of view (2 to 3 degrees), focal-plane 
arrays of large- for mat charge-coupled device (CCD) detectors, and automated signal processing 
and detection systems that recognize the asteroids and comets from their motion against the 
background of stars. The technology for such automated survey telescopes has been developed 
and demonstrated by the 0.9-m Spacewatch telescope of the University of Arizona. For purposes 
of this study, we focus on a Spaceguard Survey network of six 2.5-m aperture, f 12 prime focus 
reflecting telescopes, each with four 2048x2048 CCD chips in the focal plane. 

Follow-up and Coordination. In addition to the discovery and verification of new Earth- 
approaching asteroids and comets, the Spaceguard Survey program will require follow-up 
observations to refine orbits, determine the sizes of newly discovered objects, and establish the 
physical properties of the asteroid and comet population. Observations with large planetary 
radars are an especially effective tool for the rapid determination of accurate orbits. Radar data 
will be required to ascertain whether potentially hazardous objects will miss the Earth or, if this 
is not the case, to determine the exact time and location of the impact. Desirable for this program 
would be increased access to currently operational planetary radars in California and Puerto 
Rico, and provision of a suitable southern-hemisphere radar in the future. Although one or more 
dedicated follow-up telescopes would greatly improve our ability to study faint and distant 
asteroids and comets, we anticipate that much of the optical follow-up work can be accomplished 
with the survey telescopes themselves if they are suitably instrumented. The survey program also 
requires rapid international electronic communications and a central organization for coordi- 
nation of observing programs and maintenance of a database of discovered objects and their 
orbits. 

Expected Survey Results. Numerical modeling of the operation of the Spaceguard Survey 
network indicates that about 500 ECAs will be discovered per month. Over a period of 25 years 
we will identify more than 90 percent of potentially threatening ECAs larger than 1 km in 
diameter; a dark-sky survey will detect most incoming comets several months before they 
approach the Earth. At the same time, tens of thousands of smaller asteroids (down to a few 
meters in diameter) will also be discovered, although the completeness of the survey declines 
markedly for objects smaller than about 500 m. The advantage of this survey approach is that 
it achieves the greatest level of completeness for the largest and most dangerous objects; however, 
if continued for a long period of time, it will provide the foundation for assessing the risk posed 
by smaller impacts as well. Continued monitoring of the sky will also be needed to provide an alert 
for potentially hazardous long-period comets. 

Cost of the Spaceguard Survey. The survey can begin with current programs in the United 
States and other countries, which are providing an initial characterization of the ECApopulation 
and can serve as a testbedfor the technologies proposed for the new and larger survey telescopes. 
A modest injection of new funds into current programs could also increase current discovery rates 
by a factor of two or more, as well as provide training for personnel that will be needed to operate 
the new survey network. For the new telescopes, we assume the use of modern technology that 
has, over the past decade, substantially reduced the construction costs of telescopes of this 
aperture. The initial cost to build six 2.5-m telescopes and to establish a center for program 
coordination is estimated to be about $50M (FY93 dollars), with additional operating expenses 
for the network of about $10M per year. If construction were begun in FY93, the survey could be 
in operation by about 1997. 

Conclusions. The international survey program described in this report can be thought of as 
a modest investment to provide insurance for our planet against the ultimate catastrophe. The 
probability of a major impact during the next century is very small, but the consequences of such 
an impact, especially if the object is larger than about 1 km diameter, are sufficiently terrible to 
warrant serious consideration. The Spaceguard Survey is an essential step toward a program 
of risk reduction that can reduce the risk of an unforeseen cosmic impact by more than 75 percent 
over the next 25 years. 

vi 



CONTENTS 

Executive Summary v 

1 Introduction 1 

1.1 Background 1 

1.2 The International NEO Detection Workshop 2 

1.3 Approach to the Problem 3 

2 Hazard of Cosmic Impacts 7 

2.1 Introduction 7 

2.2 The Relationship of Risk to Size of Impactor 7 

2.3 Threshold Size for Global Catastrophe 10 

2.4 Risk Analysis H 

2.5 Conclusions 13 

3 The Near-Earth-Object Population 15 

3.1 Introduction 15 

3.2 Asteroids and Comets in Near-Earth Space 15 

3.3 Origin and Fate of NEOs 17 

3.4 Physical Properties of NEOs 17 

4 History and Current Programs 21 

4.1 Introduction 21 

4.2 Photographic Search Programs 21 

4.3 The Spacewatch CCD Scanning Program 22 

4.4 Potential of Current Programs 23 

5 Search Strategy 27 

5.1 Introduction 27 

5.2 Population Statistics of NEOs 27 

5.3 Spatial and Sky-Plane Distributions of NEOs 28 

5.4 Modeling Whole-Sky Surveys 28 

5.5 Search Area and Location 30 

5.6 Discovery Completeness 31 

5.7 Simulated Survey Scenarios 33 

5.8 Practical Considerations in Search Strategy 34 



Vll 



6 Follow-up Observations 35 

6.1 Introduction 35 

6.2 Recognition and Confirmation 35 

6.3 Optical Astrometry 36 

6.4 Radar Astrometry 37 

6.5 Physical Observations 38 

6.6 Survey Clearinghouse and Coordination Center 39 

7 Proposed Search Program 41 

7.1 Introduction 41 

7.2 Lessons from the Spacewatch Program 41 

7.3 Detector and Telescope Systems 41 

7.4 Magnitude Limit and Observing Time 42 

7.5 Number of CCD Chips and Telescopes Required 42 

7.6 Scanning Regime 43 

7.7 Computer and Communications Requirements 43 

8 International Cooperation 45 

8.1 The Necessity of International Cooperation 45 

8.2 Current International Efforts 45 

8.3 Funding Arrangements 46 

8.4 International Sanction 46 

9 The Spaceguard Survey: Summary 49 

9.1 Overview 49 

9.2 Survey Network: Cost and Schedule 50 

9.3 Conclusions 52 

Appendix A. Asteroid Tables A-l 

Appendix B. Glossary B-l 

Appendix C. References C-l 



Vlll 



CHAPTER 1 
INTRODUCTION 



1.1 BACKGROUND 



The Earth resides in a swarm of asteroids and 
comets that can, and do, impact its surface (Figs. 
1-1 and 1-2). The solar system contains a long-lived 
population of asteroids and comets, some fraction of 
which are perturbed into orbits that cross the orbits 
of the Earth and other planets. Spacecraft explora- 
tion of the terrestrial planets and the satellites of the 
outer planets has revealed crater- scarred surfaces 
that testify to a continuing rain of impacting projec- 
tiles. Additional evidence concerning cosmic projec- 
tiles in near-Earth space has accumulated since the 
discovery of the first recognized Earth-crossing aster- 
oid nearly sixty years ago, and improvements in 
telescopic search techniques have resulted in the 
discovery of dozens of near-Earth asteroids and short- 
period comets each year. The role of impacts in 
affecting the Earth's geological history, its ecosphere, 
and the evolution of life itself has become a major topic 
of current interdisciplinary interest. 

Significant attention by the scientific community 
to the hazard began in 1980 when Luis Alvarez and 
others proposed that such an impact, and the result- 
ing global pall of dust, resulted in mass extinctions of 
lifeforms on Earth, ending the age of the dinosaurs 
(Alvarez and others, 1980). Additional papers and 
discussion in the scientific literature followed, and 
widespread public interest was aroused. In 1981, 
NASA organized a workshop, "Collision of Asteroids 
and Comets with the Earth: Physical and Human 
Consequences" at Snowmass, Colorado (July 13-16, 
1981). A summary of the principal conclusions of the 
workshop report appeared in the book Cosmic Ca- 
tastrophes (Chapman and Morrison, 1989a) and in a 

(a) 



VENUS SUN 

MERCURY 




presentation by Chapman and Morrison (1 989b) at an 
American Geophysical Union Natural Hazards Sym- 
posium. In response to the close passage of asteroid 
1989 FC, the American Institute of Aeronautics and 
Astronautics (AIAA, 1990) recommended studies to 
increase the detection rate of near-Earth asteroids, 
and how to prevent such objects striking the Earth. 
The AIAA brought these recommendations to the 
attention of the House Committee on Science, Space 
and Technology, leading to the Congressional man- 
date for this workshop included in the NASA 1990 
Authorization Bill. In parallel with these political 
developments, a small group of dedicated observers 
significantly increased the discovery rate of near- 
Earth asteroids and comets, and several of these 
discoveries were highlighted in the international press. 
Other recent activity has included the 1991 Interna- 
tional Conference on Near-Earth Asteroids (San Juan 
Capistrano, California, June 30 - July 3), a meeting on 
the "Asteroid Hazard" held in St. Petersburg, Russia 
(October 9-10, 1991), and a resolution endorsing in- 
ternational searches for NEOs adopted by the Inter- 
national Astronomical Union (August 1991). 

Despite a widespread perception that asteroid 
impact is a newly recognized hazard, the basic nature 
of the hazard was roughly understood half a century 
ago. In 1941 , Fletcher Watson published an estimate 
of the rate of impacts on the Earth, based on the 
discovery of the first three Earth-approaching aster- 
oids (Apollo, Adonis, and Hermes). A few years later, 
Ralph Baldwin, in his seminal book The Face of the 
Moon (1949), wrote 

...since the Moon has always been the companion 
of the Earth, the history of the former is only a para- 



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Figure 1-1. Earth resides in a swarm of comets and asteroids, as this series of plots graphically shows: (a) the locations of the 
inner planets on January 1, 1992, (b) the orbits of the 100 largest known near-Earth asteroids, and (c) composite of (a) and (b). 



CHAPTER 1 • INTRODUCTION • 1 



ORIGINAL PAGE 
BLACK AND WHITE PHOTOGRAPH 




Figure 1-2. An aerial view of Meteor Crater, Arizona, one of the Earth 's youngest impact craters. Field studies indicate that 
the crater was formed some 50,000 years ago by an iron mass(es) traveling in excess of 11 km/s and releasing 10 to 20 megatons 
of energy. The result was the formation of a bowl-shaped crater approximately 1 km across and over 200 m deep, surrounded 
by an extensive ejecta blanket. 



phrase of the history of the latter... [Its mirror on 
Earth] contains a disturbing factor. There is no 
assurance that these meteoritic impacts have all been 
restricted to the past. Indeed we have positive evidence 
that [sizeable] meteorites and asteroids still abound in 
space and occasionally come close to the Earth. The 
explosion that formed the [lunar] crater Tycho... would, 
anywhere on Earth, be a horrifying thing, almost 
inconceivable in its monstrosity. 

Watson and Baldwin (both of whom are still alive) 
were prescient, but in their time few other scientists 
gave much thought to impacts on the Earth. Recently, 
however, there has been a gestalt shift that recog- 
nizes extraterrestrial impact as a major geological 
process and, probably, an important influence on the 
evolution of life on our planet (Figs. 1-3 and 1-4). Also 
new is our capability to detect such objects and to 
develop a space technology that could deflect a poten- 
tial projectile before it struck the Earth. 



1.2 THE INTERNATIONAL NEO DETECTION 
WORKSHOP 

The United States House of Representatives, in 
its NASA Multiyear Authorization Act of 1990 (26 
September 1990), included the following language: 

"The Committee believes that it is imperative that 
the detection rate of Earth-orbit-crossing asteroids 
must be increased substantially, and that the means to 
destroy or alter the orbits of asteroids when they 
threaten collision should be defined and agreed upon 
internationally. 

"The chances of the Earth being struck by a large 
asteroid are extremely small, but since the conse- 
quences of such a collision are extremely large, the 
Committee believes it is only prudent to assess the 
nature of the threat and prepare to deal with it. We 
have the technology to detect such asteroids and to 
prevent their collision with the Earth. 



2 • CHAPTER 1 • INTRODUCTION 



BLACK AND VYH^I 



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Figure 1-3. The heavily cratered highlands of the Moon 
record the period of heavy bombardment that marked the 
first 500 million years of lunar history. 



"The Committee therefore directs that NASA un- 
dertake two workshop studies. The first would define 
a program for dramatically increasing the detection 
rate of Earth-orbit-crossing asteroids; this study would 
address the costs, schedule, technology, and equip- 
ment required for precise definition of the orbits of 
such bodies. The second study would define systems 
and technologies to alter the orbits of such asteroids or 
to destroy them if they should pose a danger to life on 
Earth. The Committee recommends international 
participation in these studies and suggests that they be 
conducted within a year of the passage of this legisla- 
tion. " 

The present report of the NASA International 
Near-Earth-Object Detection Workshop is the direct 
result of this Congressional request to NASA. A 
second NASA workshop on the question of altering 
asteroid orbits is scheduled for January 1992. 

The NASA International Near-Earth-Object De- 
tection Workshop was organized in the spring of 1 991 
and held three formal meetings: on June 30 - July 3 at 
the San Juan Capistrano Research Institute, on Sep- 
tember 24-25 at the NASA Ames Research Center, 
and on November 5 in Palo Alto, California. The 
group has the following membership of 24 individuals 
from four continents: 



Richard Binzel (Massachusetts Institute of 
Technology, USA) 

Edward Bowell (Lowell Observatory, USA) 

Clark Chapman (Planetary Science Institute, 

USA) 

Louis Friedman (The Planetary Society, USA) 

Tom Gehrels (University of Arizona, USA) 

Eleanor Helin (Caltech/NASA Jet Propulsion 
Laboratory, USA) 

Brian Marsden (Harvard-Smithsonian Center for 
Astrophysics, USA) 

Alain Maury (Observatoire de la Cote d'Azur, 
France) 

Thomas Morgan (NASA Headquarters, USA) 

David Morrison (NASA Ames Research Center, 

USA) 

Karri Muinonen (University of Helsinki, Finland) 

Steven Ostro (Caltech/NASA Jet Propulsion 
Laboratory, USA) 

John Pike (Federation of American Scientists, 

USA) 

Jurgen Rahe (NASA Headquarters, USA) 

R. Rajamohan (Indian Institute of Astrophysics, 
India) 

John Rather (NASA Headquarters, USA) 

Kenneth Russell (Anglo-Australian Observatory, 
Australia) 

Eugene Shoemaker (U.S. Geological Survey, USA) 

Andrej Sokolsky (Institute for Theoretical 
Astronomy, Russia) 

Duncan Steel (Anglo-Australian Observatory, 
Australia) 

David Tholen (University of Hawaii, USA) 

Joseph Veverka (Cornell University, USA) 

Faith Vilas (NASA Johnson Space Center, USA) 

Donald Yeomans (Caltech/NASA Jet Propulsion 
Laboratory, USA) 



1.3 APPROACH TO THE PROBLEM 

As described in the following chapters of this 
report, the workshop group has analyzed the nature 
of the hazard and devised an example of a practical 
program for the detection of potentially catastrophic 
cosmic impacts. The greatest risk is from the impact 
of the largest near-Earth Objects (NEOs) — those 



CHAPTER 1 • INTRODUCTION • 3 




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4 • CHAPTER 1 • INTRODUCTION 



with diameters greater than 1 km. Such impacts, 
which occur on average from once to several times per 
million years, are qualitatively as well as quantita- 
tively different from any other natural disasters in 
that their consequences are global, affecting the en- 
tire planet. How, then, should we approach the prob- 
lem of discovering and tracking these objects? 

About 90 percent of the potential Earth-impact- 
ing projectiles are near-Earth asteroids or short- 
period comets; the other 10 percent are intermediate- 
or long-period comets having orbital periods longer 
than 20 years. Collectively, these bodies are called 
NEOs (near-Earth objects). Most NEOs have orbits 
that closely approach or at some time intersect Earth's 
orbit, although the intermediate- and long-period 
comets spend very little time in near-Earth space. 
Their normal orbital motion brings most near-Earth 
asteroids relatively near the Earth every few years, 
and it is during such approaches that they have 
hitherto been discovered. The objective of the exten- 
sive NEO survey described here is to find most of the 
larger and potentially hazardous NEOs (not neces- 
sarily when they are near the Earth), to calculate 
their long-term orbital trajectories, and to identify 
any that may impact the Earth over the next several 
centuries. If any appear to be on Earth-impact trajec- 
tories, there will generally be a period of at least 
several decades during which to take corrective ac- 
tion. It should be emphasized that we are not dis- 
cussing either a short-range search or a quick-response 
defense system. The chance that a near-Earth asteroid 
will be discovered less than a few years before impact 
is vanishingly small. The nature of the NEO orbits 
allows us to carry out a deliberate, comprehensive 
survey with ample time to react if any threatening 
NEO is found. In contrast, however, the warning time 
for impact from a long-period comet might be as short 
as a few months, requiring a different class of re- 
sponse. 

In order to carry out a deliberate and comprehen- 
sive search, we must detect, over a period of a decade 
or more, the NEOs larger than our 1-km size thresh- 
old that pass near the Earth. This requires that we 
monitor a region of space extending outward from the 
orbit of the Earth approximately as far as the inner 



edge of the main asteroid belt, at a distance of 200 
million kilometers. The easiest way to detect these 
NEOs is by observing their reflected sunlight, al- 
though they can also be seen in the infrared using 
their emitted thermal radiation. More exotic tech- 
nologies are not appropriate; radar, in particular, is 
limited to targets relatively close to the Earth, and is 
not suited to wide-field sky searches. In principle, the 
survey could be carried out either from the ground or 
from orbit. The brightness of a 1-km NEO at 200 
million kilometers, assuming a reflectivity of 3 per- 
cent or more, corresponds to stellar magnitude 22. 
Although they are quite faint, such objects are readily 
detectable with conventional ground-based telescopes 
and can be distinguished from background stars by 
their characteristic motion. Thus there is no re- 
quirement for a more expensive space-based system. 
This brightness limit also determines the minimum 
telescope aperture of about 2 m that is required for a 
complete survey. Thus we have it within our current 
capability to construct a network of survey telescopes 
at relatively modest cost that can discover and track 
essentially all of the NEOs greater than 1 km in 
diameter. In addition, this same network of optical 
survey telescopes will be capable of detecting most 
incoming intermediate- or long-period comets and 
determining if any of them has the potential to strike 
the Earth. However, the time between detection and 
possible impact will be much shorter for the long- 
period comets, as already noted. 

The survey program described in this report has 
the potential to alter fundamentally the way we view 
the threat of cosmic impacts. To date we have talked 
about a relatively undefined threat, to be discussed in 
terms of probabilities or statistical risks. While we 
know that such impacts must take place from time to 
time, we do not know if there are any specific bodies 
in space that might impact the Earth over the next 
few centuries. If this search program is carried out, 
however, we can answer this question to about the 75 
percent confidence level. If such an object is found, 
then we can turn our attention to dealing with the 
threat it poses. In other words, we have the capability 
for a 75 percent reduction in the hazard posed by 
cosmic impacts. 



CHAPTER 1 • INTRODUCTION • 5 



BLACK ANL ^'mITL 
CHAPTER 2 
HAZARD OF COSMIC IMPACTS 



J<^h*t-H 



2.1 INTRODUCTION 

Throughout its history, the Earth has been im- 
pacted by countless asteroids and comets. Smaller 
debris continually strike Earth's upper atmosphere 
where they burn due to friction with the air (Fig. 2-1 ); 
meteors (which are typically no larger than a pea and 
have masses of about a gram) can be seen every night 
from a dark location if the sky is clear. Thousands of 
meteorites (typically a few kilograms in mass) pen- 
etrate the atmosphere and fall harmlessly to the 
ground each year. On rare occasions, a meteorite 
penetrates the roof of a building, sometimes injuring 
the occupants, although to date there are no fully 
documented human fatalities. A much larger event, 
however, occurred in 1908 when a cosmic fragment 
disintegrated in the atmosphere over Tunguska, Si- 
beria, with an explosive energy of more than 10 
megatons TNT. But even the Tunguska impactor was 
merely one of the smallest of Earth's neighbors in 
space. Of primary concern are the larger objects, at 
least one kilometer in diameter. Although very rare, 
the impacts of these larger objects are capable of 
severely damaging the Earth's ecosystem with a re- 
sultant massive loss of life. 

In the following discussion, we examine the risks 
posed by impacting objects of various sizes. These 
projectiles could be either cometary or asteroidal. In 
terms of the damage they do, it matters little whether 
they would be called comets or asteroids by astro- 
nomical observers. We term these objects collectively 
NEOs (near-Earth objects). 

Every few centuries, the Earth is struck by an 
NEO large enough to cause thousands of deaths, or 
hundreds of thousands of deaths if it were to strike in 
an urban area. On time scales of millennia, impacts 
large enough to cause damage comparable to the 
greatest known natural disasters may be expected to 
occur (Pike, 1991). Indeed, during our lifetime, there 
is a small but non-zero chance (very roughly 1 in 
1 0,000) that the Earth will be struck by an object large 
enough to destroy food crops on a global scale and 
possibly end civilization as we know it (Shoemaker 
and others, 1990). 

As described in Chapter 3, estimates of the popu- 
lation of NEOs large enough to pose a global hazard 
are reliable to within a factor of about two, although 
estimates of the numbers of smaller objects are more 
uncertain. Particularly uncertain is the significance 
of hard-to-detect long-period comets, which would 
generally strike at higher velocities than other NEOs 
(Olsson-Steel, 1987; Weissman, 1990), although as- 



PREC£DING PAGE BLANK NOT FILMED 



teroids (including dead comets) are believed to domi- 
nate the flux. However, the resulting environmental 
consequences of the impacts of these objects are much 
less well understood. The greatest uncertainty in 
comparing the impact hazard with other natural 
hazards relates to the economic and social conse- 
quences of impact-caused damage to the ecosphere. 
Little work has been done on this problem, but we 
summarize the consequences — to the degree they are 
understood — in this chapter. 

2.2 THE RELATIONSHIP OF RISK TO 
SIZE OF IMPACTOR 

Small impacting objects that produce ordinary 
meteors or fireballs dissipate their energy high in the 
upper atmosphere and have no direct effect on the 
ground below. Only when the incoming projectile is 
larger than about 10 m diameter does it begin to pose 
some hazard to humans. The hazard can be conve- 
niently divided into three broad categories that de- 
pend on the size or kinetic energy of the impactor: 

(1) Impacting body generally is disrupted before it 
reaches the surface; most of its kinetic energy is 
dissipated in the atmosphere, resulting chiefly in 
local effects. 




Figure 2-1. On August 10, 1972, an alert photographer in 
Grand Teton National Park recorded the passage of an object 
estimated at 10 m diameter and weighing several thousand 
tons. The object narrowly missed colliding with Earth's 
surface, although it burned in our atmosphere for 101 seconds 
as it travelled over 1,475 km at about 15 ktnls. 



CHAPTER 2 • HAZARD OF COSMIC IMPACTS • 7 



(2) Impacting body reaches ground sufficiently intact 
to make a crater; effects are still chiefly local, 
although nitric oxide and dust can be carried 
large distances, and there may be a tsunami if the 
impact is in the ocean. 

(3) Large crater-forming impact generates sufficient 
globally dispersed dust to produce a significant, 
short-term change in climate worldwide, in ad- 
dition to devastating blast effects in the region of 
impact. 

The threshold size of an impacting body for each 
category depends on its density, strength, and veloc- 
ity. The threshold for global effects, in particular, is 
not well determined. 

Category 1: 10 m to 100 m diameter impactors 

Bodies near the small end of this size range 
intercept Earth every decade. Bodies about 100 m 
diameter and larger strike, on average, several times 
per millennium. The kinetic energy of a 10-m projec- 
tile traveling at a typical atmospheric entry velocity of 
20 km/s is about 50 to 100 kilotons TNT equivalent, 
equal to several Hiroshima-size bombs. The kinetic 
energy of a 100-m diameter body is equivalent to the 
explosive energy of about 100 megatons, comparable 
to the yield of the very largest thermonuclear devices. 



For the 10-m projectiles, only rare iron or stony- 
iron projectiles reach the ground with a sufficient 
fraction of their entry velocity to produce craters, as 
happened in the Sikhote-Alin region of Siberia in 
1947. Stony bodies are crushed and fragmented 
during atmospheric deceleration, and the resulting 
fragments are quickly slowed to free-fall velocity, 
while the kinetic energy is transferred to an atmo- 
spheric shock wave. Part of the shock wave energy is 
released in a burst of light and heat (called a meteoric 
fireball) and part is transported in a mechanical 
wave. Generally, these 1 00-kiloton disruptions occur 
high enough in the atmosphere so that no damage 
occurs on the ground, although the fireball can attract 
attention from distances of 600 km or more and the 
shock wave can be heard and even felt on the ground. 

With increasing size, asteroidal projectiles reach 
progressively lower levels in the atmosphere before 
disruption, and the energy transferred to the shock 
wave is correspondingly greater. There is a threshold 
where both the radiated energy from the shock and 
the pressure in the shock wave can produce damage. 
An historical example is the Tunguska event of 1908 
(Fig. 2-2), when a body perhaps 60 m in diameter was 
disrupted in the atmosphere at an altitude of about 
8 km. The energy released was about 12 megatons, as 
estimated from airwaves recorded on meteorological 



Tunguska in perspective 




Xeu- York City 




Washington. DC 



Figure 2-2. On June 30, 1908, at 7:40 AM, 
a cosmic projectile exploded in the sky 
over Siberia. It flattened 2,000 square 
kilometers of forest in the Tunguska 
region. If a similar event were to occur 
over an urban area today, hundreds of 
thousands of people would be killed, and 
damage would be measured in hundreds 
of billions of dollars. 



8 • CHAPTER 2 • HAZARD OF COSMIC IMPACTS 



ORIGINAL PAGE 
BLACK AND WHITE PHOTOGRAPH 



barographs in England, or perhaps 20 megatons as 
estimated from the radius of destruction. Siberian 
forest trees were mostly knocked to the ground out to 
distances of about 20 km from the endpoint of the 
fireball trajectory, and some were snapped off or 
knocked over at distances as great as 40 km. Circum- 
stantial evidence suggests that fires were ignited up 
to 15 km from the endpoint by the intense burst of 
radiant energy. The combined effects were similar to 
those expected from a nuclear detonation at a similar 
altitude, except, of course, that there were no accom- 
panying bursts of neutrons or gamma rays nor any 
lingering radioactivity. Should a Tunguska-like event 
happen over a densely populated area today, the 
resulting airburst would be like that of a 10- to 20- 
megaton bomb: buildings would be flattened over an 
area 20 km in radius, and exposed flammable materi- 
als would be ignited near the center of the devastated 
region. 

An associated hazard from such a Tunguska-like 
phenomenon is the possibility that it might be misin- 
terpreted as the explosion of an actual nuclear weapon, 
particularly if it were to occur in a region of the world 
where tensions were already high. Although it is 
expected that sophisticated nuclear powers would not 
respond automatically to such an event, the possible 
misinterpretation of such a natural event dramatizes 
the need for heightening public consciousness around 
the world about the nature of unusually bright fire- 
balls. 

Category 2: 100 m to 1 km diameter impactors 

Incoming asteroids of stony or metallic composi- 
tion that are larger than 1 00 m in diameter may reach 
the ground intact and produce a crater. The threshold 
size depends on the density of the impactor and its 
speed and angle of entry into the atmosphere. Evi- 
dence from the geologic record of impact craters, as 
well as theory, suggests that in the average case, 
stony objects greater than 150 m in diameter form 
craters. They strike the Earth about once per 5,000 
years and — if impacting on land — produce craters 
about 2 km in diameter. A continuous blanket of 
material ejected from such craters covers an area 
about 10 km in diameter. The zone of destruction 
extends well beyond this area, where buildings would 
be damaged or flattened by the atmospheric shock, 
and along particular directions (rays)by flying debris. 
The total area of destruction is not, however, neces- 
sarily greater than in the case of atmospheric dis- 
ruption of somewhat smaller objects, because much of 
the energy of the impactor is absorbed by the ground 
during crater formation. Thus the effects of small 
crater-forming events are still chiefly local. 

Toward the upper end of this size range, the 
energy would so vastly exceed what has been studied 



in nuclear war scenarios that it is difficult to be 
certain of the effects. Extrapolation from smaller 
yields suggests that the "local" zones of damage from 
the impact of a 1 -km object could envelop whole states 
or countries, with fatalities of tens of millions in a 
densely populated region. 

Comets are composed in large part of water ice 
and other volatile s and therefore are more easily 
fragmented than rocky or metallic asteroids. In the 
size range from 100 m to approaching 1 km, a comet 
probably cannot survive passage through the atmo- 
sphere, although it may generate atmospheric bursts 
sufficient to produce local destruction. This subject 
needs additional study, requiring a better knowledge 
of the physical nature of comets. 

Category 3: 1 km to 5 km diameter impactors 

At these larger sizes, a threshold is finally reached 
at which the impact has catastrophic global conse- 
quences, although much work remains to be done 
fully to understand the physical and chemical effects 
of material injected into the atmosphere. In general, 
the crater produced by these impacts has 10 to 15 
times the diameter of the projectile; i.e., 10 to 15 km 
diameter crater for a 1-km asteroid. Such craters are 
formed on the continents about once per 300,000 
years. At impactor sizes greater than 1 km, the 
greatest hazard derives from the global veil of dust 
injected into the stratosphere. The severity of the 
global effects of large impacts increases with the size 
of the impactor and the resulting quantity of injected 
dust. At some size, an impact would lead to massive 
world-wide crop failures and consequent mass mor- 
tality, and would threaten the survival of civilization. 
At still larger sizes, even the survival of the human 
species would be put at risk. 

What happens when an object several kilometers 
in diameter strikes the Earth at a speed of tens of 
kilometers per second? Primarily there is a massive 
explosion, sufficient to fragment and partially vapor- 
ize both the projectile and the target. Meteoric phe- 
nomena associated with high-speed ejecta could sub- 
ject plants and animals to scorching heat for about 
half an hour, and a continent- wide firestorm might 
then ensue. Dust thrown up from a very large crater 
would lead to daytime darkness over the whole Earth, 
which might persist for several months. Temperatures 
could drop as much as tens of degrees Celsius. Nitric 
acid, produced from the burning of atmospheric ni- 
trogen in the impact fireball, would acidify lakes, 
soils, streams, and perhaps the surface layer of the 
oceans. Months later, after the atmosphere had 
cleared, water vapor and carbon dioxide released to 
the stratosphere would produce an enhanced green- 
house effect, possibly raising global temperatures by 
as much as 10°C above the pre-existing ambient 



CHAPTER 2 • HAZARD OF COSMIC IMPACTS • 9 



ORIGINAL PAGE 
BLACK AND WHITE PHOTOGRAPH 



temperatures. This global warming might last for 
decades, as there are several positive feedbacks: 
warming of the surface increases the humidity of the 
troposphere, thereby increasing the greenhouse ef- 
fect, and warming of the ocean surface releases car- 
bon dioxide which also increases the greenhouse effect. 
Both the initial months of darkness and cold, and then 
the following years of enhanced temperatures, would 
severely stress the environment and would lead to 
drastic population reductions of both terrestrial and 
marine life. 

2.3 THRESHOLD SIZE FOR 
GLOBAL CATASTROPHE 

The threshold size of impactor that would produce 
one or all of the effects discussed above is not accu- 
rately known. The geochemical and paleontological 
record has demonstrated that one impact (or perhaps 
several closely spaced impacts) 65 million years ago of 
a 10- to 15-km NEO resulted in total extinction of 
about half the living species of animals and plants 
(Fig. 2-3) (Sharpton and Ward, 1990). This so-called 
K-T impact may have exceeded 100 million megatons 
in explosive energy. Such mass extinctions of species 
have recurred several times in the past few hundred 
million years; it has been suggested, although not yet 
proven, that impacts are responsible for most such 
extinction events. We know from astronomical and 
geological evidence that impacts of objects with diam- 
eters of 5 km or greater occur about once every 10 to 
30 million years. 

Death by starvation of much of the world's popu- 
lation could result from a global catastrophe far less 
horrendous than those cataclysmic impacts that would 
suddenly render a significant fraction of species actu- 
ally extinct, but we know only very poorly what size 
impact would cause such mass mortality. In addition 
to all of the known variables (site of impact, time of 
year) and the uncertainties in physical and ecological 
consequences, there is the question of how resilient 
our agriculture, commerce, economy, and societal 
organization might prove to be in the face of such an 
unprecedented catastrophe. 

These uncertainties could be expressed either as 
a wide range of possible consequences for a particular 
size (or energy) of impactor or as a range of impactor 
sizes that might produce a certain scale of global 
catastrophe. We take the second approach and express 
the uncertainty as a range of threshold impactor sizes 
that would yield a global catastrophe of the following 
proportions: 

• It would destroy most of the world's food crops for 
a year, and /or 




Figure 2-3. A thin, bright layer of clay less than an inch wide 
(toward the end of the rock-hammer handle, separated from 
the thick bright sandstone by a narrow seam of coal) marks 
debris from the catastrophic event that ended the Cretaceous 
era 65 million years ago. Here the boundary is shown in an 
outcrop near Madrid, Colorado. 



• It would result in the deaths of more than a 
quarter of the world's population, and/or 

• It would have effects on the global climate similar 
to those calculated for "nuclear winter," and/or 

• It would threaten the stability and future of 
modern civilization. 

A catastrophe having one, or all, of these traits would 
be a horrifying thing, unprecedented in history, with 
potential implications for generations to come. 

To appreciate the scale of global catastrophe that 
we have defined, it is important to be clear what it is 
not. We are talking about a catastrophe far larger 
than the effects of the great World Wars; it would 
result from an impact explosion certainly larger than 
if 100 of the very biggest hydrogen bombs ever tested 
were detonated at once. On the other hand, we are 
talking about an explosion far smaller (less than 1 
percent of the energy) than the K-T impact 65 million 
years ago. We mean a catastrophe that would threaten 
modern civilization, not an apocalypse that would 
threaten the survival of the human species. 

What is the range of impactor sizes that might 
lead to this magnitude of global catastrophe? At the 
July 1991 Near-Earth Asteroid Conference in San 
Juan Capistrano, California, the most frequently dis- 
cussed estimate of the threshold impactor diameter 
for globally catastrophic effects was about 2 km. An 
estimate of the threshold size was derived for this 
Workshop in September 1 991 by Brian Toon, of NASA/ 
Ames Research Center. Of the various environmental 
effects of a large impact, Toon believes that the great- 
est harm would be done by the sub-micrometer dust 
launched into the stratosphere. This very fine dust 



10 • CHAPTER 2 • HAZARD OF COSMIC IMPACTS 



has a long residence time, and global climate model- 
ing studies by Covey and others (1990) imply signifi- 
cant drops in global temperature that would threaten 
agriculture worldwide. The quantity of sub-microme- 
ter dust required for climate effects equivalent to 
those calculated for nuclear winter is estimated at 
about 10,000 Teragrams (Tg) (1 Tg = 10 12 g). For a 
30 km/s impact, this translates to a threshold impact- 
ing body diameter of between 1 and 1.5 km diameter. 

The threshold for an impact that causes wide- 
spread global mortality and threatens civilization 
almost certainly lies between about 0.5 and 5 km 
diameter, perhaps near 2 km. Impacts of objects this 
large occur from one to several times per million 
years. 

2.4 RISK ANALYSIS 

If this estimate of the frequency of threshold 
impacts is correct, then the chance of an asteroid 
catastrophe happening in the near future — while 
very low — is greater than the probability of other 
threats to life that our society takes very seriously. 
For purposes of discussion, we adopt a once-in-500,000- 
year estimate for the globally catastrophic impact. It 
is important to keep in mind that the frequency could 
be greater than this, although probably not by more 
than a factor often. The frequency could equally well 
be a factor often smaller. 

Because the risk of such an impact happening in 
the near future is very low, the nature of the impact 
hazard is unique in our experience. Nearly all hazards 
we face in life actually happen to someone we know, or 
we learn about them from the media, whereas no large 
impact has taken place within the total span of human 
history. (If such an event took place before the dawn 
of history roughly 10,000 years ago, there would be no 
record of the event, since we are not postulating an 
impact nearly large enough to produce a mass ex- 
tinction that would be readily visible in the fossil 
record). But also in contrast to more familiar disas- 
ters, the postulated impact would produce devasta- 
tion on a global scale. Natural disasters, including 
tornadoes and cyclones, earthquakes, tsunamis, vol- 
canic eruptions, firestorms, and floods often kill thou- 
sands of people, and occasionally several million. But 
the civilization-destroying impact exceeds all of these 
other disasters in that it could kill a billion or more 
people, leading to as large a percentage loss of life 
worldwide as that experienced by Europe from the 
Black Death in the 14th century. It is this juxtaposi- 
tion of the small probability of occurrence balanced 
against the enormous consequences if it does happen 
that makes the impact hazard such a difficult and 
controversial topic. 



2.4.1 Frequency of impacts of different sizes 

We begin to address the risk of cosmic impacts by 
looking at the frequency of events of different magni- 
tudes. Small impacts are much more frequent than 
large ones, as is shown in Fig. 2-4. This figure 
illustrates the average interval between impacts as a 
function of energy, as derived from the lunar crate ring 
record and other astronomical evidence. For purposes 
of discussion, we consider two cases: the threshold 
globally catastrophic impact discussed above, and for 
comparison, a Tunguska-class impact from a smaller 
object perhaps 100 m in diameter. In all of the 
examples given below, the numbers are approximate 
and are used only to illustrate the general magni- 
tudes involved. 

For the globally catastrophic impact: 

• Average interval between impacts: 500,000 years 

For the Tunguska-class impacts: 

• Average interval between impacts for total Earth: 
300 years 

• Average interval between impacts for populated 
area of Earth: 3,000 years 

• Average interval between impacts for world urban 
areas: 100,000 years 

• Average interval between impacts for U. S. urban 
areas only: 1,000,000 years 



Monthly 

Every Year 
u) 

t- 
o 

a Every Decade 



Once a 
Century 

Once a 
Millennium 

Every Ten 
Thousand Yrs. 

Every 100 
Thousand Yrs. 

Every 
Million Yrs. 

Every 
10 Million Yrs. 



i — i 1 — i 1 1 — i r 

"ANNUAL EVENT" -20 kllotons 




1000 YEAR EVENT" 
50 megatons 



GLOBAL 

CATASTROPHE 

THRESHOLD 

__l L_ 



1 



100 



10,000 



MEGATONS TNT EQUIVALENT ENERGY 



Figure 2-4. Estimated frequency of impacts on the Earth from 
the present population of comets and asteroids, and evidence 
from lunar craters. The megaton equivalents of energy are 
shown, as are possible and nearly certain thresholds for 
global catastrophe, (based on Shoemaker 1983) 



CHAPTER 2 • HAZARD OF COSMIC IMPACTS • 11 



We see from this simple calculation that even for 
a large, urbanized country such as the U. S., the 
Tunguska-class impacts on metropolitan areas occur 
less often than the globally catastrophic impact, em- 
phasizing the fact that the large impacts dominate 
the risk. This point is also made in Fig. 2-5, which 
plots the expected fatalities per event as a function of 
diameter (and energy) of the impacting object. The 
figure shows schematically the transition in expected 
fatalities per impact event that takes place as the 
global threshold is reached for objects between 0.5 
and 5 kilometers in diameter. 

2.4.2 Annual risk of death from impacts 

One way to address the risk is to express that risk 
in terms of the annual probability that an individual 
will be killed as a result of an impact. This annual 
probability of mortality is the product of (a) the prob- 
ability that the impact will occur and (b) the probabil- 
ity that such an event will cause the death of any 
random individual. 

For the globally catastrophic impact: 

• Average interval between impacts for total Earth: 
500,000 years 

• Annual probability of impact: 1/500,000 

• Assumed fatalities from impact: one-quarter of 
world population 

• Probability of death for an individual: 1/4 

• Annual probability of an individual's death: 
1/2,000,000 



10*° 



| 10»- 

UJ 



10-' 



CHANCE PER YEAR 

10-» 



10-6 



10» 



1Q6 



o 

ui io< 

tL 
X 



102 



WORLD POPULATION 



_L 



MEGATONS 01 




20m 



200 m 
DIAMETER OF OBJECT 



2 km 



20 km 



Figure 2-5. Large impacts dominate the risk, as seen in this 
schematic indication of expected fatalities per event as a 
function of diameter (and energy) of the impacting object. 
(C. Chapman) 



For the Tunguska-class impact: 

• Average interval between impacts for total Earth: 
300 years 

• Assumed area of devastation and total mortality 
from impact: 5,000 sq km (1/10,000 of Earth's 
surface) 

• Annual probability of an individual's death: 
1/30,000,000 

Thus we see that the annualized risk is about 1 5 
times greater from the large impact than from the 
Tunguska-class impact. 

2.4.3 Equivalent annual deaths as a measure 
of risk 

An alternative but equivalent way to express the 
risks is in terms of average annual fatalities. While 
such an index is convenient for comparison with other 
risks, we stress the artificiality of applying this ap- 
proach to the very rare impact catastrophes. The 
concept of equivalent annual deaths strictly applies 
only to averages over long periods of time in a static 
world in which the population and the mortality rate 
from other causes do not vary with time. This figure 
is obtained by multiplying the population of the Earth 
by the total annual probability of death calculated 
above. In the case of the U. S. -equivalent deaths, we 
allow for the higher-than-average population density 
in the U.S.: 

For the globally catastrophic impact: 

• Total annual probability of death: 1/2,000,000 

• Equivalent annual deaths for U. S. population 
only: 125 

• Equivalent annual deaths (worldwide population): 
2,500 

For the Tunguska-class impact: 

• Total annual probability of death: 1/30,000,000 

• Equivalent annual deaths for U. S. population 
only: 15 

• Equivalent annual deaths (worldwide population): 
150 

These figures can be compared with the mortality 
rates from other natural and human-made causes to 
obtain a very rough index of the magnitude of the 
impact-catastrophe hazard. For example, the U. S. 
numbers can be compared with such other causes of 
death as food poisoning by botulism (a few per year), 
tornadoes (100 per year), and auto accidents (50,000 
per year). 



12 • CHAPTER 2 • HAZARD OF COSMIC IMPACTS 



2.4.4 Qualitative difference for the impact 
catastrophe 

The above analysis is presented to facilitate com- 
parison of impact hazards with others with which we 
may be more familiar. However, there is a major 
qualitative difference between impact catastrophes 
and other more common natural disasters. By defini- 
tion, a global impact catastrophe would lead to a 
billion or more fatalities and an end to the world as we 
know it. No other natural disasters, including the 
much-smaller Tunguska-class impacts, have this 
nature. They represent just one among many causes 
of human death. In contrast, the potential conse- 
quences of a large impact set it apart from any other 
phenomenon with the exception of full-scale nuclear 
war. 

2.5 CONCLUSIONS 

The greatest risk from cosmic impacts is associ- 
ated with asteroids a few kilometers in diameter; such 
an impact would produce an environmental catastro- 
phe that could lead to more than a billion fatalities. 
We do not know the threshold diameter at which the 
impact effects take on this global character, but it is 
probably near 2 km, and it is unlikely to be less than 
1 km. As a first step toward significant reduction of 
this hazard, we need to identify potential asteroidal 
impactors larger than 1 km diameter. In addition, 



attention should be given to the inherently more 
difficult problem of surveying as many potential co- 
metary impactors of similar equivalent energy as is 
practical. As noted in Chapter 5, the comets account 
for 5 to 10 percent of impactors in this size range. 
However, because of their greater impact speeds, 
these comets could contribute as much as 25 percent 
of the craters larger than 20 km in diameter. 

Finally, because of the higher frequency and 
nonetheless significant consequences of impact of 
objects with diameters in the range of 100 m to 1 km, 
the survey should include bodies in this size range as 
well. There are wide differences among people in 
their psychological and political responses to hazards 
of various types. We have concentrated on the globally 
catastrophic case because of its qualitatively dreadful 
nature. But some people consider the threat of the 
more frequent Tunguska-like events to be more rel- 
evant to their concerns, even though the objective 
hazard to human life is less. To protect against such 
events (or at least mitigate their effects), impactors as 
small as 1 00 m diameter would need to be located with 
adequate warning before impact to destroy them or at 
least evacuate local populations. As described in 
Chapter 7, the survey network designed to detect and 
track the larger asteroids and comets will also discover 
tens of thousands of Earth-approaching objects in the 
100-m to 1-km size range. 



CHAPTER 2 • HAZARD OF COSMIC IMPACTS • 13 



CHAPTER 3 
THE NEAR-EARTH-OBJECT POPULATION 



3.1 INTRODUCTION 

There are two broad categories of objects with 
orbits that bring them close to the Earth: comets and 
asteroids. Asteroids and comets are distinguished by 
astronomers on the basis of their telescopic appear- 
ance. If the object is star-like in appearance, it is 
called an asteroid. If it has a visible atmosphere or 
tail, it is a comet. This distinction reflects in part a 
difference in composition: asteroids are generally 
rocky or metallic objects without atmospheres, whereas 
comets are composed in part of volatiles (like water 
ice) that evaporate when heated to produce a tenuous 
and transient atmosphere. However, a volatile-rich 
object will develop an atmosphere only if it is heated 
by the Sun, and an old comet that has lost much of its 
volatile inventory, or a comet that is far from the Sun, 
can look like an asteroid. For our purposes, the 
distinction between a comet and an asteroid is not 
very important. What matters is whether the object's 
orbit brings it close to the Earth — close enough for a 
potential collision. 

The most useful classification of NEOs is in terms 
of their orbits. The near-Earth asteroids are catego- 
rized as Amors, Apollos, and Atens, according to 
whether their orbits lie outside that of the Earth, 
cross that of the Earth with period greater than 
1 year, or cross that of the Earth with period less than 
1 year, respectively (see the Glossary for precise 
definitions of these and other technical terms). An- 
other class of NEO, consisting of asteroids and comets 
whose orbits lie entirely within the orbit of Earth, 
doubtless exist, although no such objects are currently 
known. Cometary objects are classed as short period 
if their periods are less than 20 years, intermediate 
period if their periods are between 20 and 200 years, 
and as long period if their periods are greater than 200 
years. 

Even more relevant to this report is the definition 
of an Earth-crossing asteroid (ECA). These are the 
asteroids that have the potential to impact our planet. 
An ECA is defined rigorously (Helin and Shoemaker, 
1979; Shoemaker, 1990) as an object moving on a 
trajectory that is capable of intersecting the capture 
cross-section of the Earth as a result of on-going long- 
range gravitational perturbations due to the Earth 
and other planets. In this case "long-range" refers to 
periods of tens of thousands of years. For any particu- 
lar NEO, it will not be clear whether it is in fact an 
ECA until an accurate orbit is calculated. Thus the 
concept of an ECA does not apply to a newly discov- 
ered object. Ultimately, however, it is only ECAs that 



concern us in a program aimed at discovering poten- 
tial Earth impactors. In an analogous way, we define 
Earth-crossing comets (ECCs) as intermediate- and 
long-period comets with orbits capable of intersecting 
the capture-cross-section of the Earth. 

3.2 ASTEROIDS AND COMETS IN NEAR- 
EARTH SPACE 

In 1989 there were 90 known ECAs (Shoemaker 
1990), while 128 ECAs were known at the time this 
Workshop convened in June 1991 (Appendix A). None 
of them is today a hazard, since none is currently on 
an orbit that permits collision with the Earth. But all 
of them are capable of evolving into Earth-impact 
trajectories over the next few thousand years. And, in 
fact, it is estimated that 20 to 40 percent of the ECAs 
will ultimately collide with our planet (Wetherill, 
1979; Shoemaker and others, 1990). The others will 
either be ejected from the inner solar system through 
a close encounter with the Earth or will impact or be 
ejected through close encounters with the planets 
before they reach the Earth. 

The 128 known ECAs are comprised of 11 Atens 
(9 percent), 85 Apollos (66 percent), and 32 Earth- 
crossing Amors (25 percent). Sixty-one of these have 
received permanent catalog numbers, implying their 
orbits are well established, while moderately reliable 
orbits are in hand for 51 others. The remaining 16 are 
considered lost, meaning their orbits are not well 
enough known to predict the current locations of these 
bodies. Further observations of them will occur only 
through serendipitous rediscovery. 

All ECAs brighter than absolute magnitude 13.5 
are believed to have been discovered. (The absolute 
magnitude is defined as the apparent magnitude the 
object would have if it were 1 Astronomical Unit (AU), 
or 150 million kilometers, from both the Earth and 
Sun). Translated to sizes, this means all ECAs larger 
than 14 km have been detected for the case of low 
reflectivity (dark) bodies, such as C-class asteroids. 
The limiting diameter is about 7 km for more reflec- 
tive objects, such as S-class asteroids. We estimate 
that about 35 percent of the ECAs having absolute 
magnitudes brighter than 1 5.0 (6 and 3 km diameters, 
respectively, for the dark and bright cases) have been 
discovered. At absolute magnitude 16 (4 and 2 km), 
the estimated completeness is only 15 percent, while 
at absolute magnitude 17.7 (2 and 1 km), it is only 
about 7 percent. The largest ECAs are 1627 Ivar and 
1580 Betulia, each with diameter of about 8 km, or 
slightly smaller than the object whose impact ended 



e *®t~ii 



PRECEDING PAGE BLANK NOT FILMED 



CHAPTER 3 • THE NEAR-EARTH-OBJECT POPULATION • 15 



the Cretaceous period. The smallest ECAs yet discov- 
ered are 1 991 BA, an object that passed within 0. 001 1 
AU (one-half the distance to the Moon) in January 
1 991 , and 1 991 TU, which passed within .0049 AU in 
October 1991; both have diameters of about 10 m. 

Based on search statistics and the lunar cratering 
record, we estimate that the populations of Earth- 
crossing asteroids and comets can be approximated by 
several power laws, which reflect a general exponential 
increase in the numbers of NEOs as we go to smaller 
and smaller sizes. Each segment of the distributions 
can be described, mathematically, as follows, where N 
is larger than a given diameter D: 

N = kD b 

where k is a constant and b is the power-law exponent. 
Although the general form of the size distributions for 
asteroids and comets is demonstrated by observa- 
tions, the detailed distributions are not accurately 
known. The simulations that will be described in 
subsequent chapters require models for the asteroid 
and comet populations, however. For our ECA 
population model, we estimate that changes in the 
power law occur at diameters of 0.25 and 2.5 km, and 
have adopted exponents of -2.6 (D < 0.25 km), -2.0 
(0.25 km < D < 2.5 km), and -4.3 (D > 2.5 km). 

Estimates for the total number of asteroids hav- 
ing diameters larger than values of particular inter- 
est are shown in Fig. 3-1 by the solid curve. Specific 
population estimates at sizes of interest are indicated 
in the figure, where our uncertainties are bounded by 
the dashed lines. For example, we estimate there are 
2,100 ECAs larger than 1 km in diameter, with an 
uncertainty of a factor of two. 

Active comets can also cross the Earth's orbit with 
the potential for collision. From Everhart's (1967) 
determination of cometary orbits, it can be inferred 
that 10 to 20 percent of all short-period comets are 
Earth-crossing. Using this fraction and the size- 
frequency distribution of short-period comets derived 
by Shoemaker and Wolfe (1982), we estimate that the 
population of short-period comets having Earth- 
crossing orbits is likely to comprise about 30 ±10 
objects larger than 1 km diameter, 125 ±30 larger 
than 0.5 km diameter, and 3000 ±1000 larger than 
0.1 km diameter. Comparing these numbers with 
those for the ECA population in Fig. 3-1 shows that at 
any given size, short-period comets contribute only an 
additional 1 percent or so to the total population. This 
contribution is negligible compared to the estimated 
uncertainty in the ECA population. As stated previ- 
ously, an object that displays no apparent atmosphere 
or tail is classified as an asteroid even if its orbital 
properties are similar to that of a short-period comet. 
Dormant or extinct short-period comet nuclei are 







I I I I | I II 


I | I I I I | 


I I I I | II II | I I 


Tl 




10 


~ 10m 






- 




8' 


U1 50 million 






- 






,- N. 






- 


«r 












TJ 












o 
























» 


6 


— > 


•v 100 m 




— 


< 






N^320,000 




_ 


O 
























X 




— 






— 


E 












3 

2 

o 








500 m 




4 







"^9,200 


— 


0) 








\. 1 km 




o 




— 




>S*2,100 








- 




^v 2 km 

V 400 


- 






~ 










2 


- 






- 







I I I I I I I I 


I I II I I I 


i i i i I i l l l I i i 


I II 



-1.5 -1 -0.5 

log 10 (Diameter, km) 



0.5 



Figure 3-1. Estimated number of Earth-crossing asteroids 
larger than a given diameter (E. Bowell). 



therefore likely members of the ECA population, and 
such objects are implicitly included in the ECA esti- 
mates given above. 

Although about 700 long-period comets are known 
to have passed through the inner solar system during 
recorded history, their total population is difficult to 
characterize. Only about half of these comets had 
Earth-crossing orbits and thus can be termed ECCs, 
where we define a comet to be an ECC if it has period 
greater than 20 years and a perihelion less than 1 .01 7 
AU. Fernandez and Ip (1991) estimate a flux of about 
three ECCs brighter than absolute magnitude of 10.5 
per year. From work by Weissman (1991), we esti- 
mate these bodies to be between 3 and 8 km in 
diameter. From their orbital and size distributions, 
we estimate that ECCs are about five times more 
abundant than Earth-crossing short-period comets. 
Thus the total number of ECCs is only about 5 to 10 
percent that of the ECA population. As noted previ- 
ously, however, the long-period comets contribute 
disproportionately to the impact flux because of their 
higher impact speeds relative to those of the asteroids. 
Indeed, we estimate that they contribute about 25 
percent of the total NEO hazard. To model the flux of 
ECCs that move inside the Earth's orbit, we assume 



16 • CHAPTERS • THE NEAR-EARTH-OBJECT POPULATION 



a power-law distribution of 180 D (1 97) per year. This 
flux appears to be two or three times larger than 
others have estimated because our model associated 
a larger nucleus diameter with a given apparent 
brightness, but the predicted number of ECCs of a 
given brightness should remain unaffected. 

3.3 ORIGIN AND FATE OF NEOs 

Near-Earth objects are efficiently removed from 
the solar system by collisions or gravitational interac- 
tions with the planets on time-scales of 10 to 100 
million years. Thus the NEO population we see today 
must be continually resupplied, as any remnant pri- 
mordial population would have long been depleted. 
This process of depletion has had consequences for the 
geological evolution of the terrestrial planets, as evi- 
denced by the existence of large craters. Removal of 
NEOs by impacts has profound consequences for 
biological evolution on Earth. 

As the basis for understanding the origin of NEOs 
is the need to identify their source of resupply 
(Wetherill, 1979). Cometary objects appear to be 
supplied from either the very distant reservoir called 
the Oort cloud or the somewhat closer disk called the 
Kuiper belt, which have preserved unprocessed 
(unheated) material from the time of the solar system's 
formation. The great age and primitive chemistry of 
comets make their study vital to our understanding of 
planetary accretion and chemistry. Galactic tidal 
effects and random gravitational perturbations from 
passing stars or molecular clouds can alter the orbits 
of Oort cloud members, causing some of them to make 
a close approach to the Sun. Although the comets 
initially have long orbital periods, they can be per- 
turbed into short-period orbits through interactions 
with Jupiter and the other planets. 

Two sources have been hypothesized for supply- 
ing asteroidal NEOs, both with profound implications 
on our understanding of solar system evolution. The 
first hypothesis is that they are derived from main- 
belt asteroids through the process of collisions and 
chaotic dynamics. It has been shown that objects 
orbiting in a 3:1 mean motion resonance with Jupiter 
(the location of one of the "Kirkwood Gaps" at 2.5 AU) 
exhibit chaotic increases in their orbital eccentricity, 
allowing their orbits to cross those of the terrestrial 
planets. In addition to the dynamical calculations 
that support this hypothesis, observational evidence 
shows that many NEOs are compositionally similar to 
main-belt asteroids. In many ways, they seem to 
resemble the smaller main-belt asteroids, and both 
theory and observation support the hypothesis that 
both groups consist primarily of fragments generated 
in occasional collisions between main-belt asteroids. 



A second proposed source for NEOs is from dor- 
mant or extinct comet nuclei. The end stages of a 
comet's life are poorly understood; one scenario is that 
as surface volatiles are depleted, an inert mantle 
forms which effectively seals off and insulates volatiles 
within the interior. Without the presence of an 
atmosphere or tail, such a body would have an aste- 
roidal appearance. Observational evidence that sup- 
ports this hypothesis includes several asteroidal NEOs 
that have orbits similar to known short-period com- 
ets. At least one of the cataloged asteroids, 3200 
Phaethon, is known to be associated with a strong 
meteor stream (the Geminids). Previously, strong 
meteor streams were known to be associated only 
with active comets. Further, the orbits of some as- 
teroidal NEOs do not appear to follow strict gravita- 
tional dynamics, suggesting the action of some non- 
gravitational forces such as those associated with 
cometary activity. 

3.4 PHYSICAL PROPERTIES OF NEOs 

The physical and compositional nature of aster- 
oids and comets is inferred from telescopic observa- 
tions aided by comparisons with the meteorites. 
Most meteorites appear to be fragments of asteroids, 
and in many cases it is possible to match the reflec- 
tance spectra of individual asteroids with those of 
meteorites measured in the laboratory (Fig. 3-2). Most 
of this work has been done for the main-belt asteroids, 
however, since the near-Earth asteroids are generally 
faint and must be observed within a rather narrow 
window of accessibility. 



ASTEROIDS 



METEORITES 




0.5 1.0 1.5 2.0 2.5 0.5 1.0 1.5 2.0 2.5 

WAVELENGTH (nm) 

Figure 3-2. Comparison of the spectral reflectance of 
asteroids and meteorites (C. Chapman). 



CHAPTER 3 • THE NEAR-EARTH-OBJECT POPULATION • 17 



ORIGINAL PAGE 
BLACK AND WH|T€ PHOTOGRAPH 





Figure 3-3. The Apollo asteroid 4769 Castalia is shown in the discovery photo at left taken on August 9, 1989 using the 0.46-m 
Schmidt telescope at the Palomar Observatory. Quick alerts allowed follow-up by radar observations (right) on August 22 at 
Arecibo, Puerto Rico. Radar images revealed the asteroid's two-lobed form and its four-hour rotation rate. (Photographs 
courtesy of (left) E. F. Helin (Caltech/JPL), Planet-Crossing Asteroid Survey; (right) S. Ostro (CaltechlJPL). 



Although most known Earth-approaching aster- 
oids have never been observed for physical properties, 
and those that have been are generally only poorly 
observed relative to the brighter main-belt asteroids, 
some things can be said about them. They exhibit a 
diversity in inferred mineralogy approaching that in 
the rest of the asteroid population. The majority are 
expected to be similar to the dark C-type asteroids in 
general properties (presumably moderately low-den- 
sity, volatile-rich bodies, colored black due to at least 
several percent of opaque material). There are also a 
large number of S-types. (S-types are thought to be 
either stony, chondrite-like objects, stony-iron ob- 
jects, or a combination of both.) In addition, there are 
known examples of metallic bodies (probably like 
nickel-iron alloy meteorites) and basaltic bodies. 

These asteroids are small and often quite irregu- 
lar in shape; they also tend to have rather rapid spins, 
but there is a great diversity in such properties. Their 
densities have not been measured, but are inferred to 
be typical of rocky material (about 2 to 3 g/cm 3 ). In 
only one case has an Earth-approaching object been 
imaged: 4769 Castalia (Fig. 3-3). Remarkably, the 
radar image shows a highly elongated object that may 
be a contact binary composed of two objects of compa- 
rable size. Although astronomers have presumed 
that these objects are coherent, intact bodies like 
large boulders, it is possible that some or many of 



them are aggregates, like rubble piles, which may 
have little or no internal cohesion. 

Only one asteroid has been investigated by a 
spacecraft: in October 1991, the Jupiter-bound Gali- 
leo spacecraft passed within 1,600 km of the main- 
belt asteroid 951 Gaspra(Fig. 3-4). Gaspra, an irregu- 
larly shaped S-type asteroid, is slightly larger than 
the largest known ECAs. 

It is particularly uncertain what the physical 
properties of comets (dead or alive) might be like. 
Only one comet has been studied in detail: Comet 
Halley , which was the target of several flyby spacecraft 
missions at the time of its last apparition in 1986. The 
nucleus of Halley (Fig. 3-5) is irregular and dark, with 
an average diameter of about 10 km. Like other 
comets, it is made of a combination of ice(s), rocks, and 
dust. Much of the atmospheric outgassing near the 
Sun is confined to discrete plumes or jets. In general, 
the physical configuration of comets is even less well 
understood than that of the small asteroids, and 
many comets have been observed to split under rather 
modest tidal and thermal forces. Their densities have 
not been measured but are thought to be about 1 g/ 
cm 3 , although many different estimates can be found 
in the scientific literature on comets. If we assume 
that comets are homogeneous and have roughly the 
same composition as Halley, then cometary nuclei are 
about half non-volatiles and half ices by volume. The 
non-volatiles include both silicates and organic mate- 



18 • CHAPTER 3 • THE NEAR-EARTH-OBJECT POPULATION 



BLACK AND WHITE PHOTOGRAPH 




Figure 3-4. 951 Gaspra, an S-type main-belt asteroid, was 
imaged by the Jupiter-bound Galileo spacecraft on October 
29, 1991 from a distance of about 16,200 km. Gaspra is an 
irregularly shaped object measuring about 18x11x10 km. It 
is the only asteroid yet studied by a spacecraft. 



Figure 3-5. The nucleus of Comet Halley, as seen from the 
European Space Agency's Giotto spacecraft. 



rials. The primary ices (with percentages derived for 
Halley) are water (80 percent) and carbon monoxide 
(15 percent), plus lesser quantities of formaldehyde, 
carbon dioxide, methane, and hydrocyanic acid. 

The relationships among the brightness of com- 
ets, the size of their solid nuclei, and their distance 
from the Sun are complex and not fully understood. 
Two comets with known nuclear sizes (both about 10 
km diameter), Halley and IRAS-Araki-Alcock, dif- 
fered by more than a factor of 100 in intrinsic bright- 
ness when near 1 AU from the Sun. Each well- 
observed intermediate- or long-period comet has ex- 
hibited a different pattern of activity as it approached 
and retreated from perihelion. Indeed, periodic comets 
exhibit different patterns of activity on different re- 
turns. Though seldom observed at solar distances 
greater than 5 AU, most long-period comets evidently 
become active somewhere between 5 and 10 AU. 

For a study of impacts, it is not essential to know 
a great deal about the physical nature of comets and 



asteroids. The most important properties are simply 
their mass and impact velocity, although it would 
make a difference if the projectile were double or 
multiple and easily came apart as it entered the 
atmosphere. Any future program for intercepting and 
diverting an incoming comet or asteroid will require 
detailed knowledge of the configuration, density, co- 
hesion, and composition of these objects. For these 
reasons, in addition to their significance for basic 
science, spacecraft missions to comets and near-Earth 
asteroids are essential. The first opportunity for a 
detailed study of a comet is provided by the NASA 
Comet Rendezvous and Asteroid Flyby mission 
(CRAF), now planned to study Comet Kopff in 2006- 
09. The opportunity for a similar study of a near- 
Earth asteroid will depend on approval of the NASA 
Discovery program of small planetary missions, the 
first of which is to be a rendezvous with a near-Earth 
asteroid. 



CHAPTER 3 • THE NEAR-EARTH-OBJECT POPULATION • 19 



CHAPTER 4 
HISTORY AND CURRENT PROGRAMS 



4.1 INTRODUCTION 

The first recognized Earth-crossing asteroid, 
Apollo, was discovered photographically in 1932 at 
Heidelberg and then lost until 1973. In the following 
decades only a handful of additional ECAs were dis- 
covered, and many of these were temporarily lost 
also. Not until the 1970s was a regular search initi- 
ated, using a wide-field Schmidt telescope of modest 
aperture. Several expanded photographic survey 
programs continue today with steadily increasing 
discovery rates. In the early 1980s these photographic 
approaches were supplemented by a new technique of 
electronic CCD scanning implemented at the Univer- 
sity of Arizona, and by the late 1980s this more 
automated approach was also yielding many new 
discoveries. Even today, however, the total worldwide 
effort to search for NEOs amounts to fewer than a 
dozen full-time-equivalent workers, a number of whom 
are volunteers! In this chapter we briefly review the 
history and current status of both the photographic 
and CCD searches. 

4.2 PHOTOGRAPHIC SEARCH PROGRAMS 

The overwhelming majority of discoveries of near- 
Earth asteroids (and increasingly of comets) has been 
obtained from photographic searches carried out with 
wide-field Schmidt telescopes. The bulk of discoveries 
have been made in the last decade, and the rate of 
discovery is rapidly increasing. This increase is due 
in part to improved technology but principally to 
increased interest within the astronomical community. 

To date the two most productive photographic 
teams in this field have been those directed by E. F. 
Helin and E. M. Shoemaker. Most of their work has 
been done using the 0.46-m Schmidt telescope at 
Palomar Observatory, California. Observing programs 
on three large Schmidt telescopes located in France, 
Chile, and Australia have also contributed but rather 
sporadically, as has work carried out with a narrower- 
field astrograph in Ukraine. A new successful pro- 
gram has recently been started on the U. K. Schmidt 
in Australia. The three main photographic programs 
now in operation are described briefly below. 

Various techniques are used to detect and mea- 
sure NEOs , but the search process must be carried out 
very soon after the exposure in order to permit rapid 
follow-up. In some programs the films are exposed in 
pairs with a gap in time between the first and subse- 
quent exposure, then scanned with a specially built 
stereo comparator. Images that move noticeably 

PRECEDING PAGE BLANK NOT FILMED 



between the first and second exposure may be de- 
tected in this way. Alternatively, a visual search can 
be carried out using a binocular microscope, and 
trailed images (produced by the motion of the NEO 
during the time exposure) are noted. The angular 
velocity may be inferred from the motion between 
exposures or, in the case of a single exposure, from the 
trail length (Fig. 4-1). Selection of potential NEOs is 
carried out on the basis of this angular velocity, and 
only those objects with anomalous motions are fol- 
lowed up to determine precise orbits. 

A variety of photographic emulsions have been 
used in NEO searches, but the most effective have 
been the Ilia-type emulsions coated on glass from 
Kodak, introduced twenty years ago, and a panchro- 
matic emulsion coated on a film base released in 1 982, 
again from Kodak. The new film (4415) has been 
particularly useful and is now the emulsion of choice 
for this work. 

Planet-Crossing Asteroid Survey (PCAS) 

The PCAS survey for Earth-crossing and other 
planet-crossing asteroids was initiated by E. F. Helin 
and E. M. Shoemaker in 1973 and is now directed by 




Figure 4-1. 2062 Aten, discovered in January 1976, was the 
first asteroid found with an orbit smaller than Earth 's orbit, 
and is the prototype of Aten asteroids. (E. F. Helin) 



^"^*^iW|.lfl-: 







' tiwiia UaKnA tite^gj^ 



CHAPTER 4 • HISTORY AND CURRENT PROGRAMS • 21 



Helin (Helin and Shoemaker, 1979; Helin and Dunbar, 
1984, 1990). It is the longest-running dedicated search 
program for the discovery of near-Earth asteroids and is 
carried out with the 0.46-m Schmidt telescope at Palomar 
Observatory in California. Early in the survey, about 
1,000 square degrees of sky were photographed each 
month. In the last ten years, the use of fast film has 
allowed shorter exposures leading to greater sky cover- 
age. This fact, in combination with a custom-made 
stereo-microscope, has resulted in a five-fold increase in 
the discovery rate over the early years of the program. 
Using the stereo pair method, up to 4,000 independent 
square degrees of sky can be photographed per month. 
This program has been particularly successful in getting 
out early alerts on new discoveries so physical observa- 
tions can be obtained at other telescopes during the 
discovery apparition. There has also been an organized 
international aspect to this program, called the Interna- 
tional Near-Earth Asteroid Survey (INAS), which at- 
tempts to expand the sky coverage and the discovery and 
recovery of NEAs around the world. 

Palomar Asteroid and Comet Survey (PACS) 

A second survey with the Palomar 0.46-m Schmidt 
was begun by E. M. and C. S. Shoemaker in 1982 and 
has continued with the collaboration of H. E. Holt and 
D. H. Levy (Shoemaker and others, 1990). About 
3,000 square degrees of sky are photographed each 
month. Both the PACS and PCAS programs center 
their sky coverage at opposition and along the ecliptic 
and attempt to cover as much sky as possible in every 
seven-night observing run at the telescope. The two 
programs combined produce about 6,000 independent 
square degrees of sky coverage per month. 

Anglo-Australian Near-Earth Asteroid Survey 

(AANEAS) 

The AANEAS program began in 1990 under the 
direction of D. I. Steel with the collaboration of R. H. 
McN aught and K. S. Russell using a visual search of 
essentially all plates taken with the 1.2-m U. K. 
Schmidt Telescope as part of the regular sky survey 
(Steel and McNaught, 1991). Up to 2,500 square 
degrees are covered each month to a limiting stellar 
magnitude near 22. 

4.3 THE SPACEWATCH CCD SCANNING 
PROGRAM 

An alternative to photographic search programs 
was developed at the University of Arizona under the 
name "Spacewatch" by T. Gehrels in collaboration 
with R. MacMillan, D. Rabinowitz, and J. Scotti 
(Gehrels, 1991; Rabinowitz, 1991). This system 
makes use of a CCD detector instead of photographic 
plates. It differs from the wide-field Schmidt searches 
in scanning smaller areas of sky but doing so to 



greater depth. In 1 981 , the Director of the University 
of Arizona Observatories made the Steward 0.9-m 
Newtonian reflector on Kitt Peak available, and ini- 
tial funding for instrument development was obtained 
from NASA. By 1983 Spacewatch had a 320 x 512 
pixel CCD in operation, which was too small for 
discovery of near-Earth asteroids on that telescope, 
but was exercised in order to get experience with CCD 
modes of operation. Later this was upgraded to a 
2048x2048 pixel CCD. 

The basic construction and operation of the CCD 
are ideal for scanning. We refer to the "scanning 
mode"; in older literature it is called Time Delay 
Integration (TDI). The scanning is done by exactly 
matching the rate of transfer of the charges, from row 
to row of the CCD chip, with the rate of scanning by 
the telescope on the sky. A basic advantage of scan- 
ning is the smooth continuous operation, reading the 
CCD out during observing, compared to a stop-and-go 
procedure resetting the telescope for each exposure 
and waiting for the CCD to be read out before the next 
exposure can be started. Another advantage of scan- 
ning is that the differences in pixel sensitivity are 
averaged out, and two-dimensional "flat fielding" 
calibration is therefore not needed. 

As each line of the CCD image is clocked into the 
serial shift register, it is read out by the microcomputer 
and passed on to the workstation. There the data are 
displayed, searched for moving objects, and recorded on 
magnetic tape. As each moving object is discovered 
(Fig. 4-2), from the three repeated scan regions of about 
30 minutes length, its image is copied to a separate 
"gallery" window for verification by the observer. Some 
five years of computer programming went into this 
system. 




Figure 4-2. Discovery image of near-Earth asteroid 1990 SS, 
discovered on September 25, 1990 by the Spacewatch CCD- 
scanning system. A main-belt asteroid is on the left and the 
faster-moving 1990 SS is on the right. The bright squares in 
each case indicate the positions on the previous two scans. 



22 • CHAPTER 4 • HISTORY AND CURRENT PROGRAMS 



Currently this Spacewatch system is discovering 
approximately as many NEOs as the photographic sur- 
veys. As a consequence of its more sensitive detector, it 
also tends to discover more smaller objects, including 
three objects found in 1991 that are only about 10 m in 
diameter. Substantial increases in capability are pro- 
posed with a new telescope of larger aperture (1 .8 m) to 
replace the current Spacewatch telescope in the same 
dome. 

4.4 POTENTIAL OF CURRENT PROGRAMS 

Later chapters of this Report describe a survey 
program based on a new generation of scanning tele- 
scopes. However, there is still excellent work to be 
done with current instruments during the transition 
to the new survey. The near-term potential of photo- 
graphic techniques may be considered in the following 
context. With the provision of about $1 million capital 
costs and $1 million per year operating expenses it 
would be possible to boost the current worldwide 
photographic discovery rate by at least a factor of two. 
Similarly, an upgrade of the Spacewatch CCD scan- 
ning system to 1 .8-m aperture would more than double 
the output of this system, and still greater gains are 
possible utilizing advanced, large-format CCDs. This 
instrument can also be used as a test-bed for new NEO 



survey techniques such as use of CCD arrays, opti- 
mizing of scanning strategies, and refinement of au- 
tomated search software. 

By the time large search telescopes with CCD 
detectors become available later in this decade it 
would be possible to have a sample of at least 1,000 
NEOs with well-determined orbits. From this sample, 
which should include about 10 percent of the larger 
bodies, we will gain a much better idea of the physical 
properties and dynamical distribution of the total 
population. Such information will be invaluable in 
optimizing the search strategy of the large new tele- 
scopes. In addition, the operation of the large CCD 
search facilities will require trained personnel and a 
complex organization to utilize them to the fullest 
extent, and expansion of current programs can pro- 
vide the experienced staff that will be required if and 
when the full survey begins operation. 

We assume in the following facility overview that 
wide-field photography will continue in a substan- 
tially productive manner for a number of years. CCD 
work is expected at the Spacewatch telescope on Kitt 
Peak in Arizona (with proposed upgrade to 1.8-m 
aperture) and with the French OCA Schmidt and the 
Palomar 0.46-m Schmidt, both of which are proposed 
for conversion to CCD operation. 



CHAPTER 4 • HISTORY AND CURRENT PROGRAMS • 23 



CURRENTLY ACTIVE AND POTENTIAL NEW PROGRAMS 



PALOMAR OBSERVATORY, CALIFORNIA: 
0.46-m Schmidt 



This telescope is already highly productive as a photographic instru- 
ment, supporting both the PCAS and PACS programs described in the text. 
For continued photographic work, the main requirement is in the area of 
running costs and relatively straightforward instrumental additions. Plans 
are also underway to convert to CCD detectors. 




PALOMAR OBSERVATORY, CALIFORNIA: 
1.2-m Oschin Schmidt 



This telescope, while currently dedicated to the new northern sky 
surveys, made significant contributions in the late '70s to mid '80s and has 
potential to make a significant contribution to asteroid searches; no specific 
plans for asteroid work are in place, however. 





KITT PEAK OBSERVATORY, ARIZONA: 
Spacewatch CCD Scanning Telescope 



This telescope presently has 0.9-m aperture, with plans to upgrade to 
1.8 m when funding is obtained. It is used for development of CCD scanning 
and data reduction techniques as well as the search for NEOs. The 
2048x2048-pixel CCD, largest in the world, is seen in a liquid-nitrogen 
cooled dewar at the top, permanently mounted at the south Newtonian port. 



ORIGINAL PAGE 
btLACK AND WHITE PHOTOGRAPH 



24 • CHAPTER 4 • HISTORY AND CURRENT PROGRAMS 



LOWELL ANDERSON MESA OBSERVATORY, 
ARIZONA: Perkins 0.4-m Schmidt 

This telescope exists but has not yet been installed 
at Anderson Mesa station. E. M. and C. S. Shoemaker 
hope to divide their time between this facility and 
Palomar if funds can be found to support its operation. 



CERRO TOLOLO INTERAMERICAN 
OBSERVATORY, CHILE: Curtis 0.6-m Schmidt 

Currently used for comet work, this telescope has 
recently been equipped with CCD detectors and could 
be used for some NEO searches or follow-up. 



SIDING SPRING, AUSTRALIA: 
UK 1.2-m Schmidt 

The last year has seen a dramatic increase in the 
discovery rate from this telescope, now operated by 
the Anglo-Australian Observatory, and additional 
upgrades are planned for the near future. However, 
it does not provide for a comprehensive NEO search 
program of the sort being pursued at Palomar and 
Kitt Peak. 



SIDING SPRING, AUSTRALIA: 
Uppsala 0.5-m Schmidt 

This telescope, owned by Uppsala Observatory, is 
currently used mainly for follow-up asteroid observa- 
tions. 



CAUSSOLS, FRANCE: 
OCA 1-m Schmidt 

Formerly called the CERGA Schmidt, this instru- 
ment is part of the Observatoire de la Cote d'Azur. 
Currently in limited use for NEO searches, this tele- 
scope is planned for conversion to CCD detectors in 
1992, with a program aiming toward a 16-chip array 
to scan a band 8 degrees wide in the sky. 



CRIMEA, UKRAINE: Crimean Astrophysical 
Observatory 0.4-m Astrograph 

NEOs have also been discovered using other pho- 
tographic telescopes, notably with the 0.4-m 
astrograph of the Crimean Astrophysical Observa- 
tory in Ukraine. This instrument has been in use 
since 1963 in a study of faint main-belt asteroids, but 
it has also yielded several new NEOs. 



EUROPEAN SOUTHERN OBSERVATORY, 
CHILE: ESO 1.0-m Schmidt 

The survey work for which this telescope was 
constructed is now complete, and it may be available 
for NE searches if suitably instrumented and funded. 



CHAPTER 4 • HISTORY AND CURRENT PROGRAMS • 25 



CHAPTER 5 
SEARCH STRATEGY 



5.1 INTRODUCTION 

It is feasible to conduct a survey for NEOs that 
will identify a large fraction of the asteroids and 
comets that are potentially hazardous to Earth (de- 
fined, for our purposes, as those that can come within 
about 0.05 AU, or about 20 times the distance to the 
Moon). Our objective in this chapter is to describe 
survey strategies that will identify a high percentage 
of potentially hazardous ECAs and short-period comets 
larger than 1 km diameter, and will provide advanced 
warning of the approach of hazardous long-period 
comets. This same survey will also yield many discov- 
eries of smaller bodies, some of which are potential 
hazards on a local or regional basis. 

A comprehensive survey requires monitoring a 
large volume of space to discover asteroids and comets 
whose orbits can bring them close to the Earth. Such 
bodies can be distinguished from main-belt asteroids 
by their differing motions in the sky and, in the case 
of comets, by visible traces of activity. To ensure 
reasonable levels of completeness, the volume within 
which we can find a 1-km or larger asteroid should 
extend as far as the inner edge of the main asteroid 
belt. Such a search could be carried out in the visible 
or infrared part of the spectrum, using telescopes on 
the Earth or in space. The analysis in this chapter is 
directed toward detection of the visible sunlight re- 
flected from these NEOs, with no distinction made 
between telescopes on the ground or in orbit. How- 
ever, since the least expensive option — ground-based 
astronomical telescopes with CCD detectors — is 
capable of meeting our survey requirements, we rec- 
ommend this simple and cost-effective approach. 

In this chapter we define a search strategy and 
use computer modeling to explore its quantitative 
implications. In Chapter 6 we describe the follow-up 
observations required to refine the orbits of newly 
discovered objects, and in Chapter 7 we present a 
proposed plan for an international network of survey 
telescopes to carry out this program. 

5.2 POPULATION STATISTICS OF NEOS 

To develop a quantitative survey strategy, we 
begin with the model for the Earth-approaching as- 
teroids and comets that was described in Chapter 3. 
Although only a small fraction of these near-Earth 
asteroids and comets is now known, we have suffi- 
cient information to characterize the population for 
purposes of search simulation. 

PRECEDING PAGE BLANK NOT FILMED 

'A. 



"Wl „,;1TI, , ft»?E?<fift»tai.> &i^*fc 



5.2.1 Asteroids 

We have used the set of 128 known ECAs (Appen- 
dix A) in carrying out search simulations. Our ob- 
jectives are defined in terms of discovery of these 
ECAs. This survey will also discover a large number 
of closely related Amor asteroids whose orbits will 
become Earth-crossing some tens or hundreds of mil- 
lions of years in the future. The survey is also 
capable of discovering small main-belt asteroids, at a 
rate about 200 times greater than that of the ECAs. 

The known ECA population is biased by observa- 
tional selection (which tends to favor objects with 
orbits that bring them often into near-Earth space) 
and by the reflectivities of the bodies' surfaces (which 
favors the detection of bright objects over dark ones). 
Muinonen and others (1991) computed encounter 
velocities and collision probabilities of individual as- 
teroids to correct for known sources of bias. The 
diameter distribution was approximated by a series of 
power laws, as described in Chapter 3. For our model 
simulation, there are 2,100 ECAs larger than 1 km 
diameter, 9,200 larger than 0.5 km, and 320,000 
larger than 0.1 km. Of those larger than 0.5 km in 
diameter, about 3 percent are Atens, 85 percent are 
Apollos, and 12 percent are Earth-crossing Amors. 
Although the total population of ECAs larger than 
1 km diameter is uncertain by as much as a factor of 
two (Fig. 3-1 ), the results of simulated surveys and the 
indications they provide about observing strategy 
should be qualitatively correct. 

5.2.2 Comets 

Since the orbits of short-period comets (those with 
periods less than 20 years) are rather similar to the 
ECAs, no special strategy need be devised to discover 
these comets. Indeed, the activity of most short- 
period comets makes them brighter and thus will 
enhance their discovery relative to ECAs of the same 
diameter. In what follows, the modeling of the dis- 
covery of ECAs should be taken to include that of 
short-period comets. 

The intermediate- and long-period comets are 
quite different from short-period comets. For purposes 
of this report, we use the term ECC (Earth-crossing 
comet) for all comets having periods greater than 20 
years and perihelion distances less than 1.017 AU. 
Because the majority of the ECCs discovered will 
make just one passage through the inner solar system 
during a survey of 25 years, they do not provide the 
repeated opportunities for discovery that exist for the 
ECAs. The best we can do is to identify incoming 

CHAPTER 5 • SEARCH STRATEGY • 27 



(a) 





Figure 5-1. (a) Positions of known ECAs on 23 September 1991 in an ecliptic north pole projection of the inner solar system. The 
direction of the vernal equinox is to the right. Orbits and locations of Earth, Mars, and Jupiter are also shown, (b) Ecliptic 
plane projections of ECAs, on the same scale as (a). The ecliptic is shown as the horizontal line. 



ECCs in time to give the longest possible warning 
time of their approach. For our simulations, we have 
used a sample of 158 ECCs observed during the last 
1 00 years. We assume that the observations represent 
an unbiased sample of the true ECC population. 
According to this model, there are about 100 ECCs per 
year larger than 1 km diameter that pass within the 
orbit of the Earth. 

In simulating the ECCs, we have also taken into 
account their activity (formation of an atmosphere), 
which causes them to brighten much more rapidly as 
they approach the Sun than would be expected from 
their size alone. The presence of an atmosphere 
enhances the detectability of comets, but the effect is 
not large until the comet comes inside the orbit of 
Jupiter, at which point we typically have only about 
one year warning. 

5.3 SPATIAL AND SKY-PLANE 
DISTRIBUTIONS OF ECAs 

Figure 5-1 shows the locations of the known ECAs 
on 23 September 1991 as seen from (a) north of the 
plane of the solar system and (b) as seen in that plane. 
About 10 percent are inside the Earth's orbit, and 
about 25 percent inside Mars'; these percentages 
should not vary much with time . Most of the E C As are 
rather distant, the median geocentric distance being 
about 2.2 AU (where 1 AU is 1 50 million kilometers or 
about 375 times the distance to the Moon). Assuming 
practical observational limits of magnitude V = 22 
and solar elongations greater than 75 deg (to be 



discussed in greater detail below), about one-third of 
the known ECAs are observable from the Earth at any 
time. 

The model population described above has been 
used to estimate the apparent or sky-plane distribu- 
tion of ECAs (Muinonen and others, 1991 ). From Fig. 
5-2, one expects a prevalence of detectable ECAs in 
the opposition and conjunction directions (that is, 
away from the Sun and toward the Sun). We also 
expect a concentration toward the ecliptic. The region 
near the Sun is not observable. These expectations 
are confirmed in Fig. 5-3, which shows instantaneous 
number-density contours of ECAs larger than 0.5 km 
diameter for limiting magnitudes V = 18, 20, and 22 
(note that larger magnitudes refer to fainter objects). 
Near opposition, and ignoring detection losses other 
than trailing produced by the apparent motion of the 
object, about 300 square degrees must be searched to 
V = 18 to be almost certain of detecting an ECA. To 
detect one at V = 20 we must search 50 square degrees, 
and 15 square degrees at V = 22. 

5.4 MODELING WHOLE-SKY SURVEYS 

To estimate the likely outcome of an ECA search 
program and to devise a sound observing strategy, 
Bowell and others (1991) used the model ECA popu- 
lation described above to simulate the results of 
10-year surveys. Their results have since been ex- 
panded to include ECCs in the simulations described 
in this report. Factors investigated are: limiting search 
magnitude; search area and location; observing fre- 



28 • CHAPTER 5 • SEARCH STRATEGY 



10 



< 



-5 



-10 



i — i — i — i — | — r 



1 — i — i — i — i — | — i — i — i — r 
V = 22 mag 



16 km 




D = 16km 



-10 



J I I L 



-5 




x(AU) 



Innermost contours: 
D = 125, 250, 500 m 

J I I I I I I I L 



10 



Figure 5-2. Detectability of dark C-class asteroids in the 
ecliptic plane as a function ofdiameter, assuming a magnitude 
limit V = 22. The effects of detection losses (see text) are not 
included. The origin of the coordinate system chosen is 
midway between the Earth and Sun, which are located at the 
ends of the dashed line. 



quency; and survey duration. The simulations not 
only predict the percentage completeness of NEO 
discovery as a function of diameter, but they also 
impose requirements on instrumentation and soft- 
ware, suggest some of the necessary capabilities of a 
global network of observing stations, and give point- 
ers on follow-up and orbit-determination strategy. 

To model the expected rate of discovery of ECAs 
and ECCs, and to understand how a survey for ECAs 
can be optimized, we have allowed for the effects of 
detection losses — that is, of factors that cause some 
objects to be missed or that reduce the probability of 
their detection. These losses include trailing (as 
noted above), confusion with main-belt asteroids, and 
confusion with stars and galaxies. So-called "picket- 
fence" losses, which occur when an NEO eludes de- 
tection because of its rapid motion across regions 
being scanned, have not been included. 

No survey can cover the entire sky because of 
interference from the Sun and Moon and other prac- 
tical considerations. But as a reference, we calculate 
the fraction of NEOs discovered in a hypothetical 
whole-sky survey as a function of diameter, limiting 
magnitude, and survey duration. Figure 5-4 illus- 
trates the results of ECA-survey simulations in which 
detection losses are allowed for and in which the 
whole sky is searched once each month. At a limiting 
magnitude of V = 18, comparable to the limit of the 



i i i i | i i i i | i i i i | i i i i I i i i \ t 

V = 18 mag * ' 

- Contours of log 10 n(ECAs'sr) 9 . 

Range of densities = . * > 

0.0 — 1.2 » \ X ♦ 



UJ 

o 



[i i i 1 
(a) 




50 



1 i 1 1 1 1 



100 150 200 250 

CELESTIAL LONGITUDE 



. . I . i ■ . I 



300 



350 



-50 



'I I I I | I I I I | 



I I | I I I I | I I I I | I 



V = 20 mag 

Contours of log 10 n(ECAs'sr) 

Range of densities = 1.0 — 2.0 

.Contour interval = 0.2 



I I | I I I I | 

(b) 




■ i 



I 1 r 



I ' 111 I 



50 



100 



150 200 250 

CELESTIAL LONGITUDE 



350 



V = 22 mag 
" Contours of log , n(ECAs/sr) 
- Range of densities = 1.6 — 2.8 

Contour Interval = 0.2 



i i i i | 



i I | i i i i | i i i i 



I ' i ' ' I 
(C) 




50 100 150 200 250 

CELESTIAL LONGITUDE 



I 



300 



350 



Figure 5-3. Modeled sky-plane number density of detectable 
ECAs larger than 0.5 km diameter for three limiting 
magnitudes: (a) V= 18, (b) V= 20, and (c) V = 22. Celestial 
longitude increases westward from conjunction (opposition 
is at longitude 180°). Contours of the logarithm of the number 
density ofECAspersteradian are shown at an interval of 0.2 
over the ranges indicated. Note that regions near the Sun 
cannot be observed. 



CHAPTERS • SEARCH STRATEGY • 29 



100 



CO 
CO 

111 



0. 

s 
o 
o 

> 
c 

Ul 

> 
o 
o 

CO 
5 

< 

u 

ui 




YEAR 

Figure 5-4. Discovery completeness of EC As resulting from 
whole-sky surveys. Four curve types pertain, from the bottom, 
to surveys having limiting magnitudes V = 18, 20, 22, and 24 
mag. For each limiting magnitude, results for two diameter 
thresholds are shown: D > 0.5 and 1 km. 



0.46-m Palomar Schmidt telescope currently used for 
several photographic surveys, even whole-sky surveys 
extending as long as 25 years yield only a small 
fraction of the largest ECAs. The problem is that the 
volume of space being searched is so small that many 
of the ECAs of interest simply do not pass through the 
volume being surveyed in a 25-year span. At V = 20, 
which is somewhat inferior to the current performance 
of the 0.9-m Spacewatch Telescope, fewer than half 
the ECAs larger than 1 km diameter are accessible in 
1 5 years. To achieve greater completeness, and there- 
fore greater levels of risk reduction, we must utilize 
larger telescopes with fainter limiting magnitudes, as 
will be described in Chapter 7. 

At fainter magnitudes, much greater complete- 
ness is attainable, and discovery is characterized by a 
rapid initial detection rate followed after some years 
by a much slower approach to completeness. To 
survey, for example, 90 percent of ECAs larger than 
1 km, a large area of the sky must be searched each 
month for a number of years to a magnitude limit of 
V = 22 or fainter. Because most of the large ECAs can 
be expected to be discovered early on, surveys lasting 



many decades or even longer are mainly valuable for 
adding to the completeness of the discovery of smaller 
ECAs (less than 1 km diameter) and for continued 
monitoring of ECCs. 

The ECCs spend almost all of their time in the 
outer solar system, and they can approach the inner 
solar system from any direction in space. They take 
about 16 months to travel from the distance of Saturn 
(9.5 AU from the Sun) to that of Jupiter (5.2 AU) and 
a little more than an additional year to reach peri- 
helion. At any time, it is estimated that at least one 
thousand ECCs are brighter than V = 22 magnitude. 

Modeling searches of the whole sky once a month 
for ECCs to magnitude limits of V = 22 and 24 reveals 
the shortness of the warning time even for faint 
limiting magnitudes. For V = 22, we would discover 
93 percent of ECCs larger than 1-km diameter with 
three months warning time, but only 16 percent with 
one year warning time. For V = 24, the corresponding 
numbers would be 97 and 72 percent. For ECCs larger 
than 0.5 km, the discovery completeness would be 85 
and 6 percent for V = 22, and 95 and 24 percent for 
V = 24. 

From these numbers, it is clear that a high discov- 
ery percentage can only be achieved for warning times 
on the order of several months, even for a very deep 
limiting magnitude of V = 24. This result confirms 
our intuition that it is much more difficult to provide 
long lead times for ECCs than for ECAs. 

5.5 SEARCH AREA AND LOCATION 

The reference case described in Section 5.4 refers 
to a hypothetical whole-sky survey. Now we turn to 
realistic search strategies. What area of sky is it 
necessary to search, and in what locations, in order to 
discover a sample of ECAs and ECCs that is reason- 
ably complete to an acceptable diameter threshold? 

First we consider searching the maximum pos- 
sible amount of dark sky. It is practicable to observe 
a region extending as much as ±120 deg celestial 
longitude from opposition and ±90 deg celestial lati- 
tude. In simulating such a survey, we include all the 
detection losses previously mentioned. Table 5-1 
shows the calculated discovery completeness for a 25- 
year monthly dark-sky survey for ECAs. For V = 22 
and all ECAs larger than 1-km diameter (potentially 
hazardous ECAs will be treated in more detail in 
Section 5.7.3), the discovery completeness would be 
very high: 95 percent. For V = 24, we would virtually 
achieve total completeness. 

Table 5-2 shows the result of a perpetual monthly 
dark-sky survey for ECCs. Now, for V = 22 and D > 1 
km, the completeness with a short warning time of 
three months is 77 percent. For V = 24, we would 



30 • CHAPTER 5 • SEARCH STRATEGY 



Table 5-1. Dark-sky survey simulations forECAs (see text for 
details). The table gives percentage discovery completeness 
for both entire population and potentially hazardous ECAs 
larger than a given diameter. 



D 


V = 


= 22 


V = 24 


50 m 


3 


5 


16 25 


0.1 km 


12 


19 


42 55 


0.5 km 


76 


84 


97 98 


1.0 km 


95 


96 


99 99 



Table 5-2. Dark-sky survey simulations for ECCs (see text). 
The percentages for zero warning time correspond to the 
overall discovery completenesses. 



D> 


Warning time 


% ECCs discovered 


(km) 


(yr) 


V = 22 


V = 24 


0.5 


0.0 


67 


88 




0.25 


54 


83 




0.5 


26 


66 




1.0 


5 


16 


1.0 


0.0 


85 


94 




0.25 


77 


92 




0.5 


59 


84 




1.0 


12 


49 


5.0 


0.0 


95 


96 




0.25 


93 


95 




0.5 


87 


90 




1.0 


68 


79 




2.0 


7 


25 


10.0 


0.0 


96 


97 




0.5 


90 


93 




1.0 


74 


88 




2.0 


18 


74 




3.0 


6 


27 



achieve 92 percent discovery completeness. In con- 
trast to ECAs there is appreciable degradation of 
discovery completeness for ECCs arising from lack of 
observation at small solar elongations and low galac- 
tic latitudes. 

Figure 5-3 indicates that a search centered on 
opposition (opposite the direction toward the Sun) is 
optimum. Surveys have been simulated that cover 
various areas of the sky and in which realistic detec- 
tion losses have been included. In particular, simula- 
tions of 25-year surveys to magnitude limit V = 22 
and for ECAs larger than 0.5-km diameter show that 
to minimize the area coverage needed to achieve a 
given discovery completeness, it is clearly advanta- 
geous to search regions spanning a broader range of 
celestial latitude than celestial longitude. The same 
strategy holds for other magnitude and diameter 
thresholds. For plausible search areas (in the range 



5,000 to 10,000 square degrees per month), one may 
anticipate about two-thirds discovery completeness 
at V = 22. However, coverage in both longitude and 
latitude must not be too small or some ECAs will pass 
through the search region undetected from one month 
to the next. 

Atens pose a special problem because some of 
them make very infrequent appearances that may 
occur far from opposition in celestial longitude. It can 
be expected that only about 40 percent of the Atens 
would be discovered in a nominal 25-year, 6,000- 
square-degree-per-month survey. The discovery rate 
could be increased to nearly 60 percent by biasing the 
search away from opposition, but at a sacrifice in the 
overall ECA discovery rate. It should be recalled that 
only eleven Atens are known, so the bias-corrected 
estimate of their true number may be substantially in 
error. 

5.6 DISCOVERY COMPLETENESS 

In what follows, it will be useful to consider a so- 
called standard survey region of 6,000 square de- 
grees, centered on opposition and extending ±30 deg 
in celestial longitude and ±60 deg in celestial latitude. 

5.6.1 Asteroids 

To increase discovery completeness for a given 
search area and minimum ECA diameter, either the 
survey must be lengthened, the sky must be searched 
more frequently, the limiting magnitude must be 
increased, or detection losses must be reduced. 

As noted above, rapid decline in the discovery rate 
of ECAs at faint magnitudes makes increasing the 
duration of the survey an ineffective strategy. For 
reference, the whole- sky survey to V = 22 and for 
diameters greater than 0.5 km could yield 71 percent 
completeness after 10 years. Even after 20 years, 
completeness would rise only to 81 percent (Fig. 5-4). 

Scanning a given region of the sky twice a month 
is likewise not very effective. For the standard 6,000- 
square-degree survey region, to V = 22 and 0.5-km 
diameter threshold, the completeness after 25 years 
would rise from 66 percent to 69 percent. Scanning 
1 2,000 square degrees once per month could lead to 72 
percent completeness. 

Figures 5-4 and 5-5 attest to the high value of 
mounting very deep surveys (that is, to very faint 
magnitude limits) for ECAs, the key factor being the 
greatly increased volume of space in which ECAs of 
given diameter can be detected. Figure 5-5 shows 
discovery completeness as functions of limiting mag- 
nitude V and diameter threshold for the standard 
survey region. At V = 20 and for diameter greater 
than 0.5 km, one can expect the standard 25-year 



CHAPTER 5 • SEARCH STRATEGY • 31 



ioor-r-r 



80 



in 
in 



O 
o 

> 

lil 

> 
o 



60 



% 40- 



< 

o 

UJ 



20 




25-YEAR SURVEY 



'V = 18 



I I I I I I I I I I ' I I ' I '' ' ' I I ' 'I I I ' '' I < 



12 3 4 5 6 

MINIMUM DIAMETER (km) 

Figure 5-5. ECA discovery completeness as functions of 
threshold diameters and limiting magnitude V for the 
standard survey region (see text). In the bias-corrected model 
population examined, several large ECAs went undetected 
throughout the survey, even at faint V. 



survey to be only 27 percent complete, whereas at 
V = 22 completeness rises to 66 percent. If the diam- 
eter threshold is 1 km, completeness should increase 
to 54 percent and 88 percent, respectively. Table 5-3 
summarizes the results from the standard 25-year 
survey for ECAs, and shows that a significant fraction 
of small ECAs could be discovered. 

Examination of the orbits of ECAs not discovered 
during simulated surveys shows, not unexpectedly, 
that most of these bodies' orbits have large semimajor 
axes, high eccentricities, and/or high inclinations such 
that either their dwell times in near-Earth space are 
brief and infrequent or they never come close to Earth 
in their present orbits. Of course, the latter class of 
ECAs poses no current hazard. This result of the 
simulations thus confirms our intuition: the survey 
preferentially discovers objects that come close to the 
Earth and therefore favors the overall objective of 
detecting the most hazardous asteroids. 

5.6.2 Comets 

No survey can aspire to completeness in the dis- 
covery of ECCs, since new comets are constantly 



Table 5-3. Standard survey simulations for ECAs (see text). 
The table gives percentage discovery completeness for both 
the entire population and potentially hazardous ECAs larger 
than a given diameter. 



V = 22 



V = 24 



50 m 


2 


4 


13 


20 


0.1 km 


9 


15 


34 


44 


0.5 km 


66 


74 


91 


93 


1.0 km 


87 


91 


96 


96 



Table 5-4. Perpetual standard survey simulations for ECCs 
(see text). The percentages for zero warning time correspond 
to the overall discovery completenesses. 



D> 


Warning time 


% ECCs discovered 


(km) 


(yr) 


V = 22 


V = 24 


0.5 


0.0 


23 


42 




0.25 


21 


40 




0.5 


9 


28 




1.0 


1 


5 


1.0 


0.0 


37 


51 




0.25 


35 


49 




0.5 


23 


39 




1.0 


3 


15 


5.0 


0.0 


54 


59 




0.25 


53 


58 




0.5 


44 


51 




1.0 


22 


34 




2.0 


3 


13 


10.0 


0.0 


57 


67 




0.5 


49 


62 




1.0 


29 


54 




2.0 


8 


40 




3.0 


4 


20 



entering the inner solar system. Results for ECCs in 
a 6,000-square-degree per month survey to V = 22 and 
24 are given in Table 5-4. As before, calculations are 
for a perpetual survey. 

The warning time used in these calculations is 
actually the time from discovery to first Earth cross- 
ing. But it is equally likely that the ECC, if it is on a 
collision course, will strike Earth on the outbound 
part of its orbit, increasing the warning by a few 
weeks. 

The overall level of completeness, without regard 
to warning time, is 37 percent at 1 km, 54 percent at 
5 km, and 57 percent at 10 km diameter. Clearly, a 
survey designed for ECAs produces inferior results 
for ECCs, although the rate of discovery of these 
comets will be much greater than that achieved by 
current surveys, which rely upon relatively small 
telescopes and visual sky-sweeping by amateur as- 



32 • CHAPTER 5 • SEARCH STRATEGY 



tronomers and miss the great majority of the smaller 
long-period comets. 

5.7 SIMULATED SURVEY SCENARIOS 

The simulations described above can be used to 
infer the nature of the observing activity during each 
monthly run of a major survey. The standard survey 
region of 6,000 square degrees per month can be 
studied for this purpose. 

5.7.1 Discovery of Very Small ECAs 

We have thus far not commented on very small 
ECAs discovered, although it is obvious that many 
tiny bodies, some just a few meters across, will be 
detected (see Tables 5-1 and 5-3). To estimate how 
many, 25-year surveys of the 320,000-member model 
population of ECAs larger than 0.1 km were simu- 
lated. From Fig. 5-6, which shows size-frequency 
distributions of ECA discoveries for various magni- 
tude limits V, it may be seen that many more ECAs 
smaller than the nominal 1-km diameter threshold 
would be discovered. For a survey to V = 22, one would 
expect about 80,000 ECA discoveries, of which 60 



R 



- 24 



< 
O 
UJ 
u. 
O 
cc 
ui 
m 
S 

3 
Z 



I I I I | I I I I | I I I I | 1 ' I I | 



I I I I" 




I I 



I I I I I 1 I I I I I I 



J_L 



I I I I I 



0.5 



1 1.5 

DIAMETER (km) 



2.5 



Figure 5-6. Size-frequency distribution of ECA discoveries for 
five magnitude limits resulting from simulated 25-year surveys 
(see text). The dottedlines are extrapolations fromnumerical 
simulations. 



percent are smaller than 0.1 km, 92 percent are 
smaller than 0.5 km, and 98 percent are smaller than 
1 km diameter. In other words, for every object 
greater than 1 km diameter discovered in the stan- 
dard survey, 50 more will be found that are smaller 
than 1 km. 

5.7.2 Monthly Discovery Rate 

What would be the discovery rate per month, 
assuming that the standard survey region of 6,000 
square degrees were scanned? Figure 5-7 indicates 
that, to V = 22, one can expect more than 500 ECA 
discoveries of all diameters during the first month. 
This high initial monthly discovery rate is expected to 
tail off by a factor of about two over the course of a 25- 
year survey. The larger ECAs are preferentially 
discovered early, so that while about 5 percent of the 
ECAs discovered will be larger than 1 km diameter at 
the beginning of the survey, only 0.1 percent of the 
discoveries will be larger than 1 km diameter after 25 
years. We estimate that ECCs larger than 0.5 km 
diameter will be discovered at a steady rate of about 
15 per month. 

5.7.3 Potentially Hazardous NEOs 

Not all NEOs pose a threat to Earth. Many of 
them are in orbits that cannot, at present, bring them 
within a distance that we should be concerned about. 
The potential threat of an ECA or ECC can be gauged 
from the minimum distance of its orbit from that of 
the Earth (it can be assumed that, at some time or 
another, most ECAs will be located near the minimum 
distance). For ECAs that are not predicted to make 
very close planetary encounters (and thus will not 
have their orbits changed abruptly), we estimate that, 
over a timespan of a few hundred years, minimum 
Earth-encounter distances will not change by more 
than ten lunar distances (0.02-0.03 AU) in response 
to planetary perturbations. Thus, we can be sure that 
ECAs whose minimum inner-planet encounter dis- 
tances are larger than, say, 0.05 AU (20 lunar dis- 
tances), will not pose a threat to Earth in the coming 
centuries. Objects with smaller encounter distances 
we regard as potentially hazardous. 

Because ECAs are preferentially observable when 
close to Earth, the completeness level for potentially 
hazardous ECAs is greater than that of the popula- 
tion as a whole. For a simulated 25-year dark-sky 
survey (Table 5-1), the discovery completeness for 
potentially hazardous ECAs larger than 1-km diam- 
eter is 96 percent, whereas it is 95 percent for the 
entire ECA population. For the standard 25-year 
survey (Table 5-3), the corresponding completenesses 
are 91 and 87 percent. For ECCs, however, the 
discovery completeness is the same as that of the total 
population (Tables 5-2 and 5-4). 



CHAPTER 5 • SEARCH STRATEGY • 33 



3.5 



I I I I | I I I I | I I I I | I I I I | I I I I 
V=22 mag 




i i i i I i i i i I i i 1 1 1 II li li i li I i i i 



10 



15 



20 



25 



YEAR 



Figure 5-7. Logarithm of the number ofECAs discovered in 
the first dark run of each year of a 25-year simulated survey 
(see text). Four histograms (upper to lower) are shown for all 
discoveries (dashed) and for diameter thresholds of 0.1, 0.5, 
and 1 km. The dashed histogram, forECAs of all diameters, 
is extrapolated. 



As in Chapter 3, we note that 75 percent of the 
NEO hazard arises from ECAs and 25 percent from 
ECCs. If we specify a three-month warning time for 
ECCs, the percentages of potentially hazardous NEOs 
discovered during a 25-year dark-sky survey to limit- 
ing magnitude V = 22 are as follows: 76 percent for 
diameters greater than 0.5 km, 90 percent for diam- 
eters greater than 1 km, and 98 percent for diameters 
larger than 5 km. The corresponding percentages for 
a standard 25-year survey are: 61 percent for NEOs 
greater than 0.5 km diameter, 77 percent for 
NEOs greater than 1 km diameter, and 88 percent for 
NEOs greater than 5 km. At the larger sizes, the 
missed NEOs are almost all comets. 



5.8 PRACTICAL CONSIDERATIONS IN 
SEARCH STRATEGY 

It is inconceivable that a fully fledged network of 
completely equipped observing stations will start op- 
eration simultaneously and at full efficiency. More 
likely, current photographic and CCD searches will be 
intensified in parallel with the development of new 
survey telescopes. There exists, therefore, an impor- 
tant opportunity to refine models of the NEO popula- 
tion and to test observing strategies. In particular, 
care should be taken to preserve the pointing histories 
of any systematic searches for NEOs so more reliable 
bias correction can be carried out as the known sample 
grows. When a full-up survey is in progress, it will be 
possible to refine the population model further. For 
example, if it is determined that Atens are more 
numerous than currently thought, an improved sur- 
vey strategy could be designed to enhance their dis- 
covery. Additional physical observations of newly 
discovered ECAs will also permit us to improve the 
model and thus develop better observing strategies. 

We have shown that potentially hazardous ECAs 
can be discovered at a sufficient rate that most of the 
larger members of the ECA population can be discov- 
ered and assessed within 25 years. By prolonging the 
survey, the inventory of smaller ECAs can be brought 
to greater completeness. Indeed, we estimate that, 
using current technology to continue the standard 
survey beyond 25 years, we would stand a better- 
than-even chance, within a few hundred years, of 
discovering and identifying the ECA that might cause 
the next Tunguska-like event. In anticipation that 
huge strides in technological development would re- 
duce this interval considerably, we can be almost 
certain that such an impactor could be identified by 
means of a prolonged telescopic search. 

Since ECCs enter the inner solar system at a 
near-constant rate, many of them for the first time, 
their potential for hazard to Earth goes on forever. 
Thus, any survey of finite duration will be destined to 
ignore about 25 percent of the potential hazard posed 
to our planet. Only by continually monitoring the flux 
of ECCs in the Earth's neighborhood can we hope to 
achieve near-complete assessment of the NEO haz- 
ard. 



34 • CHAPTER 5 • SEARCH STRATEGY 



CHAPTER 6 
FOLLOW-UP OBSERVATIONS 



6.1 INTRODUCTION 

In the previous chapter we described a search 
strategy for the discovery of NEOs. However, addi- 
tional follow-up observations are required. The un- 
certainty in the determination of the NEO orbit, and 
hence our ability to predict the object's future posi- 
tion, generally increases away from the period spanned 
by the observational data. If the positional data 
obtained during the discovery apparition are inad- 
equate, then the uncertainty in the NEO's sky posi- 
tion during the next predicted apparition may be so 
large that the NEO cannot be recovered. The problem 
can be alleviated if the object is found in the existing 
file of observations of unidentified asteroids, but the 
object must otherwise be designated as lost, and it will 
remain lost until it is accidentally rediscovered. 
Clearly, we need to acquire sufficient data to mini- 
mize such loss of newly discovered objects. 

An important part of the proposed survey in- 
volves the precise definition of NEO orbits, for this is 
a prerequisite to the identification of potentially haz- 
ardous objects. The critical first step in this process 
is to follow up each NEO discovery astrometrically; 
i.e., by tracking the object optically and/or with radar. 
Every NEO discovered should be followed 
astrometrically at least until recovery at the next 
apparition is assured. Further, we must develop 
explicit criteria for possibly hazardous ECAs, and any 
object that appears to fall into the "possibly hazard- 
ous" category on the basis of initial observations must 
be carefully tracked until an improved orbit determi- 
nation allows a rigorous judgment as to its hazard 
potential. 

In the case of an ECC, which cannot be tracked 
over several orbital periods, some uncertainty as to 
where (or even whether) it will strike the Earth may 
remain almost up to the time of impact. Smaller 
(Tunguska-class) ECAs may also require extensive 
tracking to determine their point of impact with 
sufficient accuracy (say 25 km) to permit rational 
judgments concerning countermeasures, such as the 
need to evacuate areas near the target. Finally, some 
uncertainty in the impact point will always remain 
due to lack of predictability of aerodynamic forces on 
the object in the Earth's atmosphere, especially if it 
breaks up during entry. 

Apart from the astrometric follow-up observa- 
tions, additional physical observations should be made 
to estimate the size and gross characteristics of the 
NEO. The rest of this chapter discusses various 
aspects of the follow-up process in detail. 



6.2 RECOGNITION AND CONFIRMATION 

Immediately after the discovery and verification 
of an NEO, the principal need is to secure enough 
astrometric data (observations of position and veloc- 
ity) that the orbit can be determined with some 
reasonable reliability. Modern asteroid-hunting 
practice is to measure carefully the positions of the 
objects in relation to the stars, and to do so on two 
nights in quick succession. Although the above proce- 
dure is mainly designed for main-belt asteroids, its 
general features apply equally well to NEOs. The 
principal difference is that, because of its rapid mo- 
tion, an NEO can generally be recognized as such on 
the night of its discovery, permitting the discoverer to 
plan for additional observations. In the case of an 
object moderately close to the Earth, the difference in 
perspective (parallax) arising from viewing points 
that are rotated about the center of the Earth (for 
example, at the same observatory but at times several 
hours apart) permits a rather accurate triangulation 
on the object's distance and hence contributes to the 
rapid determination of its orbit. In order not to 
interrupt the actual search process, it may be better 
to secure the additional initial-night observations 
with a different instrument or at a different site, 
although it is generally appropriate for the discoverer 
to take the responsibility for seeing that these obser- 
vations are secured. 

If an NEO is very close to the Earth, it is possible 
that enough information to compute a meaningful 
orbit can be obtained on a single night. Asteroid 1991 
BA, which was observed eight times over only a five- 
hour interval, is an excellent example of this. If an 
initially computed orbit bears a resemblance to that of 
the Earth, however, it is quite probable that the object 
is an artificial satellite. There do exist artificial 
satellites in highly eccentric orbits with apogees at 
and even beyond the orbit of the Moon. In the recent 
case of tiny NEO 1991 VG, the earthlike orbit was 
verified as more observations became available, 
thereby introducing the troublesome possibility that 
this was an uncataloged artificial object that had 
completely escaped from the Earth's gravity long ago 
but that was now returning to the Earth's vicinity. As 
the quantity of "space junk" increases, similar prob- 
lems are likely to occur. 

The majority of the ECAs discovered will be vis- 
ible only for relatively short time intervals because, 
being small, they must be close to Earth to be detect- 
able. Indeed, the simulations discussed in Chapter 5 
show that in a 25-year survey covering the standard 



CHAPTER 6 • FOLLOW-UP OBSERVATIONS • 35 



6,000-square-degree region to V = 22, the distance of 
closest approach of ECAs larger than 0.5 km diameter 
peaks at only about 50 lunar distances. The number 
of monthly observing runs during which ECAs larger 
than 0.5 km diameter can be detected in the standard 
survey region is shown as a function of limiting V in 
Fig. 6-1. At V= 18, 20, 22, and 24, the percentages of 
ECAs detected in only one run are 59, 41, 20, and 4 
percent, respectively. The median numbers of monthly 
runs in which ECAs are detectable are 1,2,4, and 9, 
respectively, although a few are reobservable almost 
30 times. At a diameter threshold of 1 km and for faint 
magnitudes, the percentages of ECAs observed in 
only one run are a factor of two smaller, and the 
median numbers of runs are increased by about 50 
percent. 

In the strategy described in Chapter 5, we did not 
directly address the use of the survey telescopes to 
obtain follow-up astrometric positions near the time 
of discovery. If follow-up observations were made out 
to, say, 60 deg longitude from opposition, the percent- 



— i — i — i — i — | — i — i — i — i — ] — i — i — i — i — | — i — i — 



3.5 



D > 0.5 km 




10 20 30 

NUMBER OF DARK RUNS OBSERVABLE 

Figure 6-1. Logarithm of the number of ECAs larger than 0.5 
km diameter discovered during a 25-year survey of the 
standard region (see text) as a function of the number of dark 
runs during which they are observable. Results for limiting 
magnitudes V=18, 20, 22, and 24 are shown. The leftmost bin, 
for zero dark runs, indicates the number of undiscovered 
ECAs: log n = 3.93 (V = 18 mag), 3.82 (20), 3.50 (22), and 2.91 
(24). 



age of ECAs larger than 0.5 km seen only once to 
V = 22 would be reduced from 20 to 12 percent. Even 
greater protection against loss would be afforded by a 
follow-up strategy in which ECAs discovered were 
reobserved as long as possible in any accessible region 
of the dark sky. The question of strategy for this 
follow-up work needs further study, with the results 
depending on the availability of other supporting 
telescopes for astrometric observations. 

Since losses after observation in one monthly run 
can be reduced to small numbers, it is probable that, 
for deep ECA surveys, follow-up can largely be ig- 
nored in favor of the linkage of detections from one 
run or one apparition to another. In general, such 
linkage can be achieved unambiguously provided ob- 
servations are not too sparse. However, care must be 
taken not to lose the very fast-moving ECAs that may 
be most hazardous to Earth. Also, because of the large 
numbers of small ECAs that will be discovered, se- 
lection must be made, at least in part, on the basis of 
the diameter threshold. Both considerations call for 
a rapid estimate of the diameters of all ECAs discovered 
near the magnitude limit. To achieve this, the observed 
brightness can be combined with the distance gauged 
by means of diurnal parallax. Preference in such 
work should be given to those objects that appear to be 
true ECAs, especially those that might pose some 
threat based on initial orbit calculation. 

6.3 OPTICAL ASTROMETRY 

For a typical bright NEO, astrometric follow-up is 
essential. Much of the follow-up astrometry is most 
conveniently and efficiently accomplished using con- 
ventional reflecting telescopes fitted with CCDs. If 
conventional reflectors are used, they should gener- 
ally be in the 1- to 2-m aperture range, although 
larger telescopes should certainly be considered for 
following up very faint discoveries. A set of semi- 
dedicated observatories is preferable to a single dedi- 
cated observatory (or one in each hemisphere), if only 
for reasons of weather and availability of observers, 
and there are certainly times when the more-or-less 
continuous coverage that may thereby be possible can 
be very useful. 

Existing facilities currently involved with 
astrometric follow-up of NEOs are listed below in 
order westward from the principal U.S. discovery 
sites (the 0.46-m Schmidt at Palomar and the 
Spacewatch 0.9-m reflector at Kitt Peak), separately 
for each hemisphere: 

Northern hemisphere: 

• Victoria, B.C., Canada (0.5-m reflector with CCD); 

• Mauna Kea, Hawaii (2. 2-m U. Hawaii reflector 
and 3-m NASA IRTF with encoders); 



36 • CHAPTER 6 • FOLLOW-UP OBSERVATIONS 



• Japan (no professional but much amateur activ- 
ity); 

• Kavalur, India (fledgling Spacewatch program); 

• Kitab, Uzbekistan, and Crimean Astrophysical 
Observatory, Ukraine (0.4-m astrographs; coordi- 
nated by the Institute for Theoretical Astronomy, 
St. Petersburg, Russia); 

• Kiel, Czechoslovakia (0.6-m Maksutov; currently 
no electronic-mail communication but should be- 
come possible via Prague); 

• Western Europe (not much professional activity, 
but possibilities at Caussols, France, 0.9-m 
Schmidt, and La Palma, Canaries, 2.2-m reflector 
with CCD); 

• Oak Ridge, Massachusetts (1.5-m reflector with 
CCD); 

• Lowell Observatory, Arizona (1.1-m and 1.8-m 
reflectors with CCD). 

Other possibilities include the 1.3-m Schmidt at 
Tautenburg, Germany, and telescopes at the Bulgar- 
ian National Observatory, but these are not currently 
involved with NEOs, and rapid communication is a 
problem. 

Southern hemisphere: 

• Mount John Observatory, New Zealand (0.6-m 
reflector, conversion to CCD in progress); 

• Siding Spring, N.S.W., Australia (U.K. 1.2-m 
Schmidt, 0.5-m Uppsala Southern Schmidt, 
1.0-m reflector with CCD); 

• Perth, Western Australia, (occasional use of 
0.3-m astrograph or 0.6-m reflector); 

• European Southern Observatory, Chile 
(occasional use of 1 .0-m Schmidt, 0.4-m astrograph 
or 1.5-m reflector). 

Also, there would seem to be a need for participa- 
tion in southern Africa and eastern South America. 

6.4 RADAR ASTROMETRY 

Radar is an essential astrometric tool, yielding 
both a direct range to an NEO and the radial velocity 
(with respect to the observer) from the Doppler- 
shifted echo (Yeomans and others 1987; Ostro and 
others 1991). Since most NEOs are discovered as a 
result of their rapid motion on the sky, these objects 
are then generally close to the Earth; radar observa- 
tions are therefore often immediately possible and 
appropriate. However, radar observations do not 
become feasible until the object's expected position 
can be refined (from optical astrometry) to better than 



about 1 arcmin, and an accuracy of 10 arcsec or better 
is preferable. A single radar detection yields 
astrometry with a fractional precision that is several 
hundred times better than that of optical astrometry, 
so the inclusion of radar data with the optical data in 
the orbit solution can quickly and dramatically re- 
duce the future ephemeris uncertainty. 

The principal radar instruments are currently 
those at Arecibo, Puerto Rico, and Goldstone, Califor- 
nia. There may also be possibilities at Effelsberg, 
Germany; Parkes, N.S.W., Australia; and Yevpatoriya, 
Ukraine. Since radars are range limited, radar- 
detectability windows are narrow, but both Arecibo 
and Goldstone are being upgraded to enlarge their 
current windows. There is a clear need for a compa- 
rable facility in the southern hemisphere, and some 
preliminary planning has been done for an "Arecibo- 
class" radio telescope in Brazil which could also be 
used as a radar. 

The inclusion of radar data in the orbital solutions 
would allow an NEO's motion to be accurately inte- 
grated forward for many decades to assess the likelihood 
of future Earth impacts. With optical data alone, such 
an assessment requires an observational span of several 
decades, which may or may not be possible from the 
inspection of old photographic plates. The addition of 
radar data to the orbital solution may allow reliable 
extrapolations of the object's motion to be made within 
only days of discovery. 

There has hitherto always been a time interval, at 
least several days long, between discovery and the 
initial radar work. If the first radar ephemeris is 
found to have very large delay or Doppler errors, the 
initial radar astrometry is used to generate a second- 
generation radar ephemeris to enable finer-precision 
delay or Doppler astrometry (by at least a factor of 
ten) than would have been possible with the first 
radar ephemeris. This bootstrapping process would 
be much more efficient than it currently is if a capa- 
bility to do the computations existed at the radio 
telescope itself. Ideally, one could input the first 
measurements of Doppler and delay into a program 
on a computer at the site, generate an improved 
ephemeris within an hour of initial detection, and 
proceed immediately to high-resolution ranging. The 
existence of on-site ephemeris-generating capability 
would be essential if the astrometry that does the 
critical shrinking of the pointing uncertainty becomes 
available at the same time as the object enters the 
radar window, or with an NEO that comes so close 
that it traverses the telescope's declination-distance 
window in one day (as did comet IRAS-Araki-Alcock 
at Arecibo in 1983). 



CHAPTER 6 • FOLLOW-UP OBSERVATIONS • 37 



6.5 PHYSICAL OBSERVATIONS 

The impact energy of an NEO that actually hits 
the Earth depends on both its velocity and its mass. 
Knowledge of the orbit provides only the velocity, not 
the mass. The latter quantity can be estimated only 
from physical observations. If astrometric observa- 
tions are made with a photometric device, such as a 
CCD, they can also provide information about the 
most basic of physical parameters, namely, the 
brightness of the object. In the case of a bright comet, 
measurements of the brightness will almost certainly 
include a strong contribution from the comet's atmo- 
sphere, whereas what is needed is isolation of the 
solid nucleus, something that can be satisfactorily 
attempted only when the comet is farther from the 
Sun. 

Although an asteroid's brightness is correlated 
with its size, the known range of asteroid surface 
reflectivities spans a factor of 20, which leads to a 
large uncertainty in the volume. The range of densi- 
ties of asteroids can be inferred from their bulk com- 
positions, which may in turn be suggested by mea- 
surements of surface composition. If only a brightness 
measurement is available, the deduced mass of the 
object, and therefore the potential impact energy, can 
be uncertain by a factor of a hundred. Additional 
uncertainty arises from the fact that asteroid 
brightnesses vary as they rotate, sometimes by more 
than a factor of five. 

Measurements of the relative reflectivity of an 
asteroid at a variety of wavelengths (its spectral 
reflectivity) can place the object in one of several 
known taxonomic classes and therefore reduce the 
uncertainty in the surface reflectivity. At the same 
time, the composition of the object is constrained, 
leading to an improved estimate of the bulk density. 
In a minimal effort, the use of three filters, appropri- 
ately chosen to sample spectral features in the ul- 
traviolet and infrared regions, should be employed. 
With additional filters, greater diagnosticity can be 
achieved, with a corresponding improvement in 
reflectivity and composition estimates. With a mini- 
mal filter set, the uncertainty in the range of potential 
impact energies can be reduced to a factor of about 
ten. 

Radar observations are the only source of spa- 
tially-resolved measurements from the ground and 
hence provide the only source of direct information 
about an NEO's shape. Moreover, radar can also 
supply constraints on size that are highly reliable if 
the echoes are strong enough. Radar also provides 
some information about the composition and rough- 
ness of an NEO's surface. 



Even single-color photometry permits a rotation 
period to be determined, and radar can then provide 
the spin-pole direction. The angular momentum of a 
potentially hazardous object can therefore be calcu- 
lated, and this may be an important consideration in 
deciding on the technique to be used for dealing with 
the hazard. In the case of a comet, the detection of 
persistent cyclic variations in the brightness of the 
condensation about a stable mean is probably an 
indication that the solid nucleus has been detected. 

That NEOs differ greatly in composition is also 
evident from a comparison of the effects of encounters. 
Although the bodies that produced Meteor Crater in 
Arizona 50,000 years ago and the Tunguska event in 
Siberia 84 years ago are both thought to have been in 
the rough size range 50 to 100 m, one produced a 
crater that is still well-preserved, while the other 
apparently exploded high above the ground, produced 
no crater, but levelled trees over a much larger area. 
Knowledge of the likely composition can also play a 
prominent role in establishing the ameliorative ac- 
tion that might be taken in the case of a predicted 
impact. 

One could argue that it is not necessary to make 
physical observations until an object on a collision 
trajectory has actually been detected. This may not be 
a prudent course of action, however, for the following 
reasons. (1) The possibility exists that there will be 
no further opportunity to study the object in question 
sufficiently in advance of a collision to provide the 
necessary information on the potential impact energy 
and on how to deal with the object. (2) Discoveries of 
NEOs are often made when they are unusually close 
to the Earth, and physical observations can be per- 
formed more efficiently and with higher precision at 
these times. (3) We need to learn more about the full 
range of NEO compositions and structural properties, 
which are poorly known at present, in order to plan 
possible strategies for deflection of these objects in 
case of a predicted impact. (4) There are significant 
scientific and possible future space exploration ben- 
efits that can result from the study of a sizable portion 
of the NEO population, including the identification of 
objects with space resource potential (substantial 
sources of water or of nickel-iron and other heavy 
metals), the providing of selection criteria for possible 
future spacecraft missions to such objects, the under- 
standing of the link between terrestrial meteorites 
and the asteroid belt, and important information 
regarding the origin (cometary versus asteroidal, for 
example) of these objects. 



38 • CHAPTER 6 • FOLLOW-UP OBSERVATIONS 



6.6 SURVEY CLEARINGHOUSE AND 
COORDINATION CENTER 

Much of the discussion in this chapter has been in 
the context of current practice for NEO discoveries. 
However, the proposed new search strategy described 
in Chapter 5 means that future NEO discoveries may 
take place up to 5 magnitudes, or 100 times, fainter 
than at present. When searches routinely reach 
magnitude 22 there should be about 500 new NEO 
candidates each month. With careful organization of 
the discovery searches, however, the astrometric fol- 
low-up data could all be obtained with the same 
telescopes involved in the discovery. In particular, 
thought should be given to ensuring that the relevant 
fields are automatically recorded with a large time 
separation on either the first or the second night in 
order to make a parallactic determination of a crude 
orbit. Month-by-month opposition scanning should 
also allow, at least in principle, the correct identifica- 
tion of subsequent images of each NEO, but in order 
to ensure success it would probably be desirable to 
perform the discovery and confirmation regimen twice 
during each monthly run. 

Bright time (that is, time when the Moon is up) on 
the discovery telescopes could also be used for physi- 
cal observations. Radar observations would presum- 
ably have to be restricted to close passages by the 
Earth. Sampling of the physical properties of the 
smaller NEOs would be important in case they are 
system atically different from those of the 1 arger NE O s 
and the main-belt asteroids. However, their faintness 
makes certain observations difficult, so that a large 
dedicated follow-up telescope with special instrumen- 
tation would prove more effective for some physical 
observations than would the survey telescopes them- 
selves. 

The dramatic increase in the rate of discovery of 
NEOs will require considerable extension of the current 
system for keeping track of these objects and dissemi- 
nating information about them. Hitherto these func- 
tions have principally been carried out by the Interna- 
tional Astronomical Union's Central Bureau for Astro- 
nomical Telegrams and Minor Planet Center, which 
since 1978 have been operating together at the 
Smithsonian Astrophysical Observatory in Cambridge, 
Massachusetts, under the direction of B. G. Marsden. 
The Minor Planet Center currently deals with asteroid 
discoveries (primarily main-belt objects) at an annual 
rate of a few thousand. With the prospect of discovering 
a thousand NEOs alone in a month, augmentation of the 
Minor Planet Center's capabilities will be necessary. 



Procedures for rapidly checking, identifying, computing 
orbits and providing appropriate ephemerides for new 
discoveries are already in place, but future enhance- 
ment will require acquisition of faster computers and 
the employment of additional personnel. The future 
NEO survey clearinghouse would also undertake the 
task of actually planning the observations at the various 
sites, collecting the observations from the sites, and 
coordinating further observations to cover fields missed 
by bad weather and to ensure proper follow-up in specific 
cases. 

Further development of procedures and construc- 
tion and maintenance of software must also be an 
important component of the work of the survey clear- 
inghouse. For comets and asteroids, the computation 
of an orbit and ephemeris should include an estimate 
of the uncertainty in the NEO's location as a function 
of time, that is, the "positional error ellipsoid" 
(Yeomans and others 1987; Muinonen and Bowell 
1992). (This is less easily done in the case of comets 
because of the existence of nongravitational effects 
that can at best be modeled in a semi-empirical 
manner.) By projecting the error ellipsoid into the 
future, one can quantify the likelihood that an NEO 
will be recoverable, and one can also assess the un- 
certainty in an Earth-asteroid distance for any future 
close approaches. Such software will also (1) help to 
expedite verification of newly discovered objects as 
NEOs, (2) provide the basis for prioritizing NEOs for 
follow-up astrometry, both to avoid losing objects and 
to optimize the use of telescope time and personnel, 
and (3) permit the reliable identification of NEOs on 
very close-approach trajectories and the appropriate 
hazard assessment. 

For each newly discovered NEO, data files will 
have to be established to catalog discovery data and 
follow-up observations, both astrometric and physi- 
cal. Orbits and associated error analyses will be 
required for each object to identify close Earth ap- 
proaches in the immediate future and to establish 
optimum observation times for securing the object's 
orbit and ensuring its recovery at subsequent obser- 
vation opportunities. Once the need for follow-up 
observations has been established and the optimal 
observation times determined, the clearinghouse 
would notify the appropriate people capable of mak- 
ing the required observations and provide them with 
all the information required to utilize efficiently the 
limited amount of available telescope time. Recently, 
a NASA center for some of these clearinghouse ac- 
tivities has been established at the Jet Propulsion 
Laboratory. 



CHAPTER 6 • FOLLOW-UP OBSERVATIONS • 39 



CHAPTER 7 
PROPOSED SEARCH PROGRAM 



7.1 INTRODUCTION 

In this chapter, we assess the instrumental re- 
quirements (telescopes, mosaics of CCD chips, com- 
puters, etc.) imposed by the observing strategy and 
follow-up research outlined in Chapters 5 and 6, and 
we comment on observational techniques and observ- 
ing network operation. We concentrate on the re- 
quirements of a survey optimized for the discovery of 
ECAs, with the understanding that slightly different 
requirements are posed by a network optimized for an 
ECC search. In order to cover the requisite volume of 
search space, the survey must achieve a stellar limit- 
ing magnitude limit of at least V = 22, dictating 
telescopes of 2- to 3-m aperture equipped with CCD 
detectors. The most efficient use of CCD detectors is 
achieved if the pixel size is matched to the apparent 
stellar image size of about 1 arcsec, thus defining the 
effective focal length for the telescopes at about 5 m. 
According to the model explored in Chapter 5, the 
area of sky to be searched is about 6,000 square 
degrees per month, centered on opposition, and ex- 
tending to ±30 deg in celestial longitude and ±60 deg 
celestial latitude. These considerations lead us to a 
requirement for multiple telescopes with moderately 
wide fields of view (at least 2 deg) and mosaics of 
large-format CCD detectors. We develop these ideas 
in this chapter to derive a proposed search program. 
This program is not unique (that is, an equivalent 
result could be obtained with other appropriate choices 
of telescope optics, focal-plane detectors, survey area, 
and locations), but it is representative of the type of 
international network required to carry out our pro- 
posed survey. 

7.2 LESSONS FROM THE SPACEWATCH 
PROGRAM 

The Spacewatch Telescope, operated at the Uni- 
versity of Arizona (see Chapters 3 and 4), is the first 
telescope and digital detector system devised to carry 
out a semi-automated search for NEOs. As such, the 
lessons learned from its development and operation 
are invaluable when considering a future generation 
of scanning instruments. The Spacewatch system 
comprises a single 2048x2048-pixel CCD chip at the 
f/5 Newtonian focus of an equatorially mounted 0.9-m 
telescope. Each pixel covers 1.2x1.2 arcsec on the sky. 
With the telescope drive turned off, the camera scans 
the sky at the sidereal rate, and achieves detection of 
celestial bodies to a limiting magnitude V = 20.5. 

PRECEDING PAGE BLANK NOT FILMED 

lj (■■ , > ..;. V --i;f-.M*f(iJf ?***->*' 



One of the important demonstrations provided by 
the Spacewatch Telescope team is that image-recog- 
nition algorithms such as their Moving Object Detec- 
tion Program (MODP) are successful in making near- 
real-time discoveries of moving objects (asteroids and 
comets). False detections are almost eliminated by 
comparing images from three scans obtained one 
after the other. At present, the Spacewatch system 
makes detections by virtue of the signal present in 
individual pixels. With the incorporation of higher- 
speed computers, near-real-time comparison of indi- 
vidual pixels to measure actual image profiles would 
lead to a great reduction in the most frequent sources 
of noise, cosmic ray hits and spurious electrical noise 
events. 

In light of the successful performance of 
Spacewatch, we have rejected a photographic survey. 
Even though sufficiently deep exposures and rapid 
areal coverage could be attained to fulfill the survey 
requirements using a small number of meter-class 
Schmidt telescopes (similar to the Oschin and U.K. 
Schmidts), there is no feasible way, either by visual 
inspection or digitization of the films, to identify and 
measure the images in step with the search. A 
photographic survey would fail for lack of adequate 
data reduction and follow-up. Future developments 
in electronics and data processing will further enhance 
the advantages of digital searches over the older 
analog methods using photography. 

7.3 DETECTOR AND TELESCOPE SYSTEMS 

The largest CCD chips readily available today 
contain 2048x2048 pixels, each about 25 micrometers 
on a side. Thus, the chips are about 5x5 cm in size. 
Quantum efficiencies have attained a peak near 80 
percent, and useful sensitivity is achievable from the 
near-ultraviolet to the near-infrared. To reach a 
limiting stellar magnitude of V = 22, we require the 
use of these CCDs at the focal plane of a telescope with 
an aperture of 2 m or larger, operated during the half 
of the month when no bright moonlight is present in 
the sky (from last quarter to first quarter phase). 

In the coming decade, we envisage a trend toward 
smaller and more numerous CCD pixels covering the 
same maximum chip area as at present. No great 
increase in spectral sensitivity can be expected. At 
the telescope, the pixel scale must be matched to the 
image scale (the apparent angular size of a stellar 
image) in good or adequate atmospheric (seeing) 
conditions. In what follows, we assume a pixel scale 



CHAPTER 7 • PROPOSED SEARCH PROGRAM • 41 



of 1 arcsec/pixel (25-micrometer/arcsec, or 40 arcsec/ 
mm), which implies a telescope of 5.2-m focal length. 
For a telescope of 2 m aperture, the focal ratio is 
f/2.6; for a 2.5-m, 02.1; and for a 3-m, f/1.7. 

A single 2048x2048 CCD chip simultaneously 
detects the signals from more than 4 million indi- 
vidual pixels. This is a very powerful data-gathering 
device, but it still falls short of the requirements for 
wide-field scanning imposed by the proposed NEO 
survey. At the prime focus of a telescope of 5.2-m focal 
length, such a CCD covers a field of view on the sky 
about one-half deg on a side. However, we wish to 
scan an area at least 2 deg across. Therefore, we 
require that several CCD chips be mounted together 
(mosaicked) in the focal plane. The mosaicking of 
CCD chips is being vigorously pursued today by as- 
tronomers; at Princeton University, for example, a 
focal plane with 32 CCDs is under development. 

Studies and planning are underway at the Uni- 
versity of Arizona for a modern 1.8-m Spacewatch 
telescope. The new telescope will be an excellent 
instrument to test and develop some of the necessary 
instrumental and strategic considerations outlined in 
this report. From the Spacewatch design consider- 
ations, it is safe to assume that 2- to 3-m-class tele- 
scopes can be built having focal lengths near 5 m and 
usable fields of view between 2 and 3 deg. Refractive- 
optics field correction is probably required, and it 
appears advantageous to locate CCD mosaics at the 
prime focus of such instruments. Here, we indicate 
telescope functional requirements but do not exactly 
specify the size or design of the proposed survey 
telescopes. 

7.4 MAGNITUDE LIMIT AND 
OBSERVING TIME 

Exceptionally fine astronomical sites have more 
than 1 ,000 hr/yr of clear, moonless observing condi- 
tions, during most of which good to adequate seeing 
prevails. More typically, 700 hr/yr of observing time 
is usable. We assume that a region of 6,000 square 
degrees is to be searched each month and that initial 
NEO detection is made by two or three scans on the 
first night. Parallactic information is derived by four 
scans on a subsequent night, and an orbit is calculated 
from observations on a third night. Thus, nine or 
more scans of the search region are needed each 
month. In a given month, follow-up will be attempted 
for some of the NEOs that have moved out of the 
search region (mainly to the west). As a working 
value, we assume that 40 hr/month per telescope are 
available for searching. 

The limiting (faintest) stellar magnitude that can 
be observed by a telescope can be determined as a 
function of the ratio of the source brightness to that of 



the sky, the number of pixels occupied by a star image, 
the pixel area, the light-collecting area of the tele- 
scope, and the effective integration time (Rabinowitz 
1991). For certain detection, the source brightness 
must be at least six times that of the sky noise. We 
have normalized to the performance of the Spacewatch 
Telescope, which achieves a stellar limit of V = 20.5 
using an unfiltered 165-sec scan at sidereal rate, and 
we have allowed for an improvement over the perfor- 
mance of that system arising from improved detector 
quantum efficiency and improved image-recognition 
algorithms. We find for the survey telescopes that a 
single CCD should be able to achieve the survey 
requirement of V = 22 with the following combina- 
tions of telescope aperture and scan speed: 



Primary 


Exposure 


Scan 


Diameter 


Time 


Rate 


(m) 


(s) 


(x sidereal) 


2.0 


21 


6 


2.5 


14 


10 


3.0 


10 


14 



7.5 NUMBER OF CCD CHIPS AND 
TELESCOPES REQUIRED 

A single 2048x2048-pixel CCD chip, having an 
image scale of 1 arcsec/pixel, can scan at 0.14 square 
degrees per minute at the sidereal rate. If 40 hr/ 
month/telescope can be allotted to searching for NEOs 
over 6,000 square degrees to a limiting stellar magni- 
tude of V = 22, and ten scans per sky region are 
required for detection and rough orbital characteriza- 
tion of an NEO, then telescopes of the apertures 
considered above have the following performance ca- 
pabilities: 



Primary 


Area/month/ 


Total number 


Diameter 


CCD 


of CCDs 


(m) 


(sq. deg) 


required 


2.0 


260 


28 


2.5 


420 


18 


3.0 


600 


13 



In computing values for the total number of CCD 
chips required in the worldwide network of telescopes 
we assume that no two CCD chips together scan the 
same region of the sky. These are minimum require- 
ments for the telescopes; in practice more scans may 
be needed for reliable automatic detection, and 



42 • CHAPTER 7 • PROPOSED SEARCH PROGRAM 



probably there will be some overlap of coverage be- 
tween telescopes. 

Searching to ±60 deg celestial latitude implies 
sky coverage, over the course of a year, at almost all 
declinations. Thus telescopes must be located in both 
hemispheres. Usable fields of view of between 2 and 
3 deg probably limit the number of CCD chips in a 
telescope's focal plane to about ten at the scales we 
have been considering. However, real-time image 
processing is simplified if each chip independently 
samples the sky. Most likely, four CCD chips/tele- 
scope can be accommodated in a linear array in the 
focal plane. Thus, it appears that seven 2-m telescopes, 
five 2.5-m telescopes, or four 3-m telescopes suffice to 
fulfill the search, follow-up, and physical observations 
requirements of the idealized 6,000-square degree 
survey. Most likely, there would remain extra obser- 
vational capability to enhance the detection rates of 
Atens and ECCs by scanning a few times per month 
outside the standard region. We note that each 
telescope must be equipped with a minimum of four 
2048x2048 CCD chips or their equivalent in light- 
collecting ability. If space remains in the focal plane, 
additional filtered CCD chips could be inserted to 
undertake colorimetry, which would give a first-order 
compositional characterization of some of the NEOs 
discovered while scanning. 

To ensure that a single-point failure due to 
weather or other adverse factors will not hamper 
effective operation of the survey network, we conclude 
that three telescopes are required in each hemisphere . 
With fewer telescopes, orbital, and perhaps parallac- 
tic, information on NEOs would be sacrificed. The 
desirability of searching near the celestial poles calls 
for at least one telescope at moderate latitude in each 
hemisphere. In summary, we propose a network of six 
2-m or larger telescopes distributed in longitude and 
at various latitudes between, say, 20 deg and 40 deg 
north and south of the equator. 

7.6 SCANNING REGIME 

At high declinations, scanning along small circles 
of declination results in curvature in the plane of the 
CCD chip, so star images do not trail along a single 
row of pixels. The problem can be avoided by scanning 
along a great circle. A good strategy would be to scan 
in great circles of which the ecliptic is a meridian (the 
pole being located on the ecliptic 90 deg from the Sun). 
Such scanning can be achieved using either equato- 
rial or altitude-azimuth telescope mounts, but is 
probably more easily and cheaply accomplished using 
an altitude-azimuth mount. In either case, field 
rotation is required, as is currently routinely used at 
the Multiple-Mirror Telescope in Arizona and other 
installations. 



At the proposed 1.8-m Spacewatch telescope, it is 
planned to make three scans of each region of the sky 
(as is currently done at the 0.9-m Spacewatch tele- 
scope). Each scan would cover 1 deg in 26 min, so the 
interval between the first and third scans is sufficiently 
long that objects moving as slowly as 1 arcmin/day can 
be detected. For the proposed NEO survey, we envis- 
age two or three longitudinal scans per sky region, 
about an hour apart. Thus, at a scan rate of 10 times 
sidereal, each scan could cover an entire strip of the 
60-deg-wide search region, with a second search strip 
being interposed before the first was repeated. We 
assume that occasional false positive detections will 
not survive scrutiny on the second night of observa- 
tion, and thus will not significantly corrupt the de- 
tection database. 

7.7 COMPUTER AND COMMUNICATIONS 
REQUIREMENTS 

Near real-time detection of faint NEOs requires 
that prodigious amounts of data processing be accom- 
plished at the telescope. The image processing rate 
scales linearly with the number of objects (NEOs, 
stars, galaxies, noise, etc.) recorded per second. The 
number of objects detected per second (the "object 
rate"), and therefore computer requirements of the 
NEO survey outlined above, can be estimated from 
the current performance of the Spacewatch Telescope. 
The computer system in use at the Spacewatch Tele- 
scope can detect up to 10,000 objects in a 165-sec 
exposure. Thus, its object rate is 60/sec. Scanning to 
V - 22 requires detection of about 30,000 objects/ 
square degree. For an image scale of 1 arcsec/pixel, 
using the scanning rates tabulated above, and allow- 
ing a ten-fold increase in computing requirements to 
perform real-time image profile analysis, we calculate 
the total network computer requirement to be 2,000 to 
3,000 times that at the Spacewatch Telescope. There- 
fore at each of six telescopes, it would be 300 to 500 
times that at Spacewatch. Such a requirement, al- 
though not easy to achieve, is possible using the 
newest generation of parallel processors. 

There are at least three levels of observational 
data storage that can be envisaged: (1) preservation 
of image-parameter or pixel data only for the moving 
objects detected; (2) preservation of image-parameter 
or pixel data for all sources detected (mostly stars); (3) 
storage of all pixel data. The first option is clearly 
undesirable, because data for slow-moving NEOs 
mistaken as stars would be lost. The first two options 
have the disadvantage that there would be no way to 
search the database, after the event, for sources 
whose brightnesses are close to the limiting magnitude 
and that would therefore have been discarded. The 
third option — the most attractive scientifically — may 



CHAPTER 7 • PROPOSED SEARCH PROGRAM • 43 



appear to result in serious problems of data storage 
and retrieval. However, we anticipate that, using 
technology shortly to be available, the third option is 
tractable. 

About 500 NEOs and one hundred thousand main- 
belt asteroids could be detected each month — about 
one detection per second of observing time. Therefore, 
only moderate-speed data communication is needed 
between observing sites and a central-processing 
facility. Careful observational planning will be 



required to ensure efficient coverage of pre- 
programmed scan patterns, to avoid unintentional 
duplication of observations, to schedule the necessary 
parallactic and follow-up observations , and to optimize 
program changes so as to maintain robustness of the 
survey in response to shutdowns. Successful operation 
of this survey system will also require the coordination 
and orbital computation capabilities of a modern 
central data clearinghouse as described in Chapter 6. 



44 • CHAPTER 7 • PROPOSED SEARCH PROGRAM 



CHAPTER 8 
INTERNATIONAL COOPERATION 



8.1 THE NECESSITY OF INTERNATIONAL 
COOPERATION 

That the hazard posed by NEOs is a problem for 
all humankind hardly needs repeating. The likelihood 
of a particular spot being the target of an impact is 
independent of its geographic position, so that we are 
all at risk. Further, each person on the face of the 
planet would be severely affected by a large impact, as 
discussed in Chapter 2. 

The problem is thus international in scope; it is 
also international in solution. To obtain the spatial 
and temporal coverage of the sky that is required by 
the search program outlined in Chapter 7, a wide 
geographical coverage of optical observatory sites is 
essential. Even if these sites were limited to six, still 
at least five countries would likely be involved directly 
as telescope hosts. However, the number of nations 
actually involved would be larger than this. If Austra- 
lia were one site then most likely the Anglo-Austra- 
lian Observatory would be the organization acting as 
host, implying British involvement. Similarly a site 
in India, where a Spacewatch-type instrument is 
currently being developed, might involve a continua- 
tion of direct U.S. collaboration. Some of the best 
observatory sites in the southern hemisphere are in 
Chile, and if plans go ahead for the development of a 
large southern radar in Brazil, again the number of 
countries increases. The need for international co- 
operation is obvious, and rapid and efficient inter- 
national communication through a central agency is 
a requirement. 

8.2 CURRENT INTERNATIONAL EFFORTS 

The independent character of the scientific en- 
deavor as well as limited funding resources has re- 
sulted in a current program to find and track NEOs 
that is quite fragmentary. Generally it has been 
possible, in recent years, for discoveries made by one 
team to be followed up by other observers, but this has 
not always been the case, allowing some newly dis- 
covered NEOs to be lost. For the program planned 
here this must not be allowed to occur, emphasizing 
the need for an international effort with close coop- 
eration and priorities to be set by a central organi- 
zation. The present level of our knowledge of NEOs 
has been possible only because of the services of the 
staff of the Central Bureau for Astronomical Telegrams 
and the Minor Planet Center (Cambridge, Massa- 
chusetts) who coordinate the analysis of discoveries of 
NEOs and make every effort to ensure that sufficient 



coverage occurs. A continuation of such a service on 
a much larger scale will be necessary if the proposed 
program is to be brought to fruition. 

There have been efforts to formally organize a 
search program on an international scale, quite apart 
from the informal links and communications made 
possible by personal contacts. The most prominent of 
these organizations has been INAS, the International 
Near-Earth Asteroid Survey, coordinated by E. F. 
Helin (Helin and Dunbar, 1984, 1990). INAS has 
resulted in increased cooperation among observato- 
ries in various countries, and hence a modest increase 
in the discovery rates. Apart from the U.S., scientists 
from the following countries have been involved in 
INAS: Australia, Bulgaria, Canada, China, Czecho- 
slovakia, Denmark, France, Germany, Italy, Japan, 
New Zealand, Russia, Sweden, Ukraine, United 
Kingdom, and Yugoslavia. 

The major thrust of INAS has been to coordinate 
the efforts of the large wide-field photographic instru- 
ments with regard to temporal and sky coverage. An 
immediate expansion of this effort can increase the 
current discovery rate, thus providing valuable infor- 
mation on the true statistical nature of the NEO 
population and associated impact hazards before the 
full network of survey telescopes becomes operational. 
Such a program will also serve as a training ground 
for new personnel and provide valuable experience 
with improved international communication and co- 
ordination. 

A Spacewatch-type telescope is currently under 
development in India with the joint support of the 
U.S. Smithsonian Institution and the Government of 
India. Another international effort is being proposed 
by the Institute for Theoretical Astronomy in St. 
Petersburg, Russia, under the direction of A.G. 
Sokolsky. This group organized an international 
conference, "The Asteroid Hazard," in October 1991, 
which endorsed the idea that NEOs "represent a 
potential hazard for all human civilization and create 
a real threat of regional catastrophes" and noted "the 
necessity of coordinated international efforts on the 
problem of the asteroid hazard." This group has asked 
the Russian Academy of Science to support the for- 
mation of an International Institute on the Problem of 
the Asteroid Hazard under the auspices of the Inter- 
national Center for Scientific Culture — World 
Laboratory, and they propose to coordinate asteroid 
searches and follow-up observations in central and 
eastern Europe. 



CHAPTER 8 • INTERNATIONAL COOPERATION • 45 



8.3 FUNDING ARRANGEMENTS 

If this international survey program is to succeed, 
it must be arranged on an inter-governmental level. 
To ensure stability of operations, the NEO survey 
program needs to be run by international agreement, 
with reliable funding committed for the full duration 
of the program by each nation involved. 

There are good reasons for the funding to be 
derived from all nations directly involved in the pro- 
gram. First, most countries usually want to provide 
for their own defense rather than to rely upon another 
or others to do this for them, so we may anticipate that 
nations in the world-wide community will wish to 
each play their own part in defending the planet. 
Second, although this program is large compared 
with present NEO search efforts, in fact it would be of 
quite a small overall budget. Thus it is possible for 
nations to make a significant contribution with little 
expense whereas it would not be possible for them to 
buy into a large space project, or even the construction 
of a ground-based 10-m-class astronomical telescope. 
For example, there is a small group in Uruguay 
that studies dynamical aspects of NEO's, and they 
could provide an essential service to the program; or 
the telescopes available for follow-up work in New 
Zealand or Romania could be utilized, and thus those 
nations could gain prestige on the international scene 
at little expense. Involvement in space programs 
(which this program is, in essence) is generally viewed 
favorably by the populace of most countries. Third, 
this program may be a significant technology driver 
for small countries, so that money spent on the in- 
vestigation and development of new technologies can 
be viewed as an investment rather than an expendi- 
ture. 

With the encouragement of the United States as 
prime mover, the funding for national sectors of the 
overall international search program should be at- 
tainable locally. For example, Australia and the 
United Kingdom, through their joint observatory in 
Australia, could immediately boost the current dis- 
covery rate to about 100 per year using existing 
equipment and technology, given supplementary 
funding from those countries of the order of $0.25 
million per year, although we would anticipate that 
this effort would be superseded by the introduction of 
CCD detectors within five years. Photographic 
searches currently being carried out in the United 
States might require a similar boost in funds, with a 
concomitant boost in discovery rate resulting, and the 
Spacewatch effort could also be significantly expanded 
by approval for the upgrade to 1.8-m aperture and 
funding to run the camera on more than eighteen 
nights per month. 



8.4 INTERNATIONAL SANCTION 

The astronomical program outlined in this report 
already has the support of various international 
bodies. There is a burgeoning awareness in the 
astronomical community that the NEO impact hazard 
is a topic that requires attention for reasons other 
than altruistic scientific pursuit. At the 1991 General 
Assembly of the International Astronomical Union 
held August 1 , in Buenos Aires, Argentina, the follow- 
ing resolution was passed: 



The XXIst General Assembly of the Interna- 
tional Astronomical Union, 

Considering that various studies have shown 
that the Earth is subject to occasional impacts by 
minor bodies in the solar system, sometimes with 
catastrophic results, and 

Noting that there is well-founded evidence 
that only a very small fraction of NEO's (natural 
Near-Earth Objects: minor planets, comets and 
fragments thereof) has actually been discovered 
and have well-determined orbits, 

Affirms the importance of expanding and sus- 
taining scientific programmes for the discovery, 
continued surveillance and in-depth physical and 
theoretical study of potentially hazardous objects, 
and 

Resolves to establish an ad hoc Joint Working 
Group on NEOs, with the participation of Com- 
missions 4, 7, 9, 15, 16, 20, 21 and 22, to: 

1. Assess and quantify the potential threat, in 
close interaction with other specialists in these 
fields, 

2. Stimulate the pooling of all appropriate re- 
sources in support of relevant national and 
international programmes, 

3. Act as an international focal point and con- 
tribute to the scientific evaluation, and 

4. Report back to the XXIInd General Assembly 
of the IAUin 1994 for possible further action. 



46 • CHAPTER 8 • INTERNATIONAL COOPERATION 



The Working Group, to be convened by A. Carusi 
of Italy, comprises the following scientists: 

A. Basilevsky (Russia) 

A. Carusi (Italy) 

B. Gustafson (Sweden) 
A. Harris (USA) 

Y. Kozai (Japan) 
G. Lelievre (France) 

A. Levasseur-Regourd (France) 

B. Marsden (USA) 
D. Morrison (USA) 
A. Milani (Italy) 

K. Seidelman (USA) 



E. Shoemaker (USA) 

A. Sokolsky (Russia) 

D. Steel (Australia/UK) 

J. Stohl (Czechoslovakia) 

Tong Fu (China) 

This Working Group was selected not only on the 
basis of the geographical spread of persons active in 
the general area, but also in terms of expertise in 
distinct areas of the necessary program (e.g., celestial 
mechanics, generation of ephemerides, physical na- 
ture of NEOs, dynamics of same, relationship to 
smaller meteoroids and interplanetary dust). Five of 
these 16 individuals are also members of the NASA 
International NEO Detection workshop, ensuring ap- 
propriate continuity of effort. 



CHAPTER 8 • INTERNATIONAL COOPERATION • 47 



CHAPTER 9 
THE SPACEGUARD SURVEY: SUMMARY 



9.1 OVERVIEW 

Concern over the cosmic impact hazard motivated 
the U.S. Congress to request that NASA conduct a 
workshop to study ways to achieve a substantial 
acceleration in the discovery rate for near-Earth as- 
teroids. This report outlines an international survey 
network of ground-based telescopes that could in- 
crease the monthly discovery rate of such asteroids 
from a few to as many as a thousand. Such a program 
would reduce the time-scale required for a nearly 
complete census of large Earth-crossing asteroids 
(ECAs) from several centuries (at the current discovery 
rate) to about 25 years. We call this proposed survey 
program the Spaceguard Survey (borrowing the name 
from the similar project suggested by science-fiction 
author Arthur C. Clarke nearly 20 years ago in his 
novel Rendezvous with Rama). 

In addition, this workshop has considered the 
impact hazards associated with comets (short-, in- 
termediate-, and long-period) and with small asteroidal 
or cometary objects in the size range from tens of 
meters to hundreds of meters. The object is not 
elimination of risk, which is impossible for natural 
hazards such as impacts, but reduction of risk. Em- 
phasis, therefore, is placed upon the greater hazards, 
in an effort to define a cost-effective risk-reduction 
program. Below we summarize our conclusions with 
respect to these three groups of objects: large ECAs, 
comets, and small (Tunguska-class) objects. 

1) Large ECAs (diameter greater than 1 km; impact 
energy greater than 100,000 megatons). These 
objects constitute the greatest hazard, with their 
potential for global environmental damage and 
mass mortality. About two thousand such objects 
are believed to exist in near-Earth space, of which 
fewer than 10 percent are now known. About a 
quarter of them will eventually impact the Earth, 
but the average interval between such impacts is 
long — about 100,000 years. While some of these 
objects may break up during entry, most will 
reach the surface, forming craters if they strike on 
the land. On average, one ECA in this size range 
passes between the Earth and the Moon every few 
decades. 

The proposed Spaceguard Survey deals effectively 
with this class of objects. Telescopes of 2- to 3-m 
aperture can detect them out to a distance of 200 
million kilometers. Since their orbits bring them 
frequently within this distance of the Earth, a 
comprehensive survey will discover most of them 



within a decade and can achieve near complete- 
ness within 25 years. Specifically, the survey 
modeled here, covering 6,000 square degrees of 
sky per month to magnitude V = 22, is calculated 
to achieve 91 percent completeness for potentially 
hazardous ECAs in 25 years. The most probable 
outcome of this survey will be to find that none of 
these objects will impact the Earth within the 
next century, although a few will need to be 
followed carefully to ensure that their orbits do 
not evolve into Earth-impact trajectories. In the 
unlikely case (chance much less than 1 percent) 
that one of these ECAs poses a danger to the Earth 
over the next century or two, there probably will 
be a warning of at least several decades to take 
corrective action to deflect the object or otherwise 
mitigate the danger. 

2) Comets. Comets with short periods (less than 20 
years) will be discovered and dealt with in the 
same manner as the ECAs described above; they 
constitute only about 1 percent of the ECA hazard 
in any case. However, comets with long periods 
(more than 20 years), many of which are entering 
the inner solar system for the first time, constitute 
the second most important impact hazard. While 
their numbers amount to only 5 to 1 percent of 
the ECA impacts, they approach the Earth with 
greater speeds and hence higher energy in pro- 
portion to their mass. It is estimated that about 25 
percent of the objects reaching the Earth with 
energies in excess of 100,000 megatons are long- 
period comets. On average, one such comet passes 
between the Earth and Moon per century, and one 
strikes the Earth every few hundred thousand 
years. 

Since a long-period comet does not (by definition) 
pass frequently near the Earth, it is not possible 
to obtain a census of such objects. Each must be 
detected on its initial approach to the inner solar 
system. Fortunately, comets are much brighter 
than asteroids of the same size, as a consequence 
of outgassing stimulated by solar heating. Com- 
ets in the size range of interest will generally be 
visible to the Spaceguard Survey telescopes by 
the time they reach the asteroid belt (500 million 
km distant), providing several months of warning 
before they approach the Earth. However, the 
short time-span available for observation will 
result in less well-determined orbits, and hence 
greater uncertainty as to whether a hit is likely; 
there is a greater potential for "false alarms" with 



•*m 



H. 




m- 



CHAPTER 9 • THE SPACEGUARD SURVEY: SUMMARY • 49 



PRECEDING PAGE BLANK NOT FILMED 



comets than asteroids. Simulations carried out 
for this report indicate that only 35 percent of 
Earth-crossing intermediate- and long-period 
comets (ECCs) larger than 1 km diameter will be 
discovered with at least three months warning 
during the course of a survey centered on opposi- 
tion and covering an area of 6,000 square degrees 
per month. By increasing the area of the survey 
to encompass the entire dark sky, as many as 77 
percent of ECCs could be detected. A further gain 
in the detection completeness of ECCs could be 
achieved by increasing the telescope aperture so 
as to reach a limiting magnitude of V = 24. Be- 
cause of the continuing hazard from Earth-cross- 
ing comets, which may appear at any time, a 
search for the cometary component of the 
Spaceguard survey should be continued even when 
the census of large Earth-crossing asteroids is 
essentially complete. 

3) Smaller Asteroids, Comets, and Meteoroids 

(diameters from about 100 m to 1 km; energies 
from 20 to 100,000 megatons). Impacts by these 
bodies are below the energy threshold for global 
environmental damage, and they therefore con- 
stitute a smaller hazard in spite of their more 
frequent occurrence. Unlike the large objects, 
they do not pose a danger to civilization. The 
nature of the damage they cause depends on the 
size, impact speed, and physical nature of the 
impacting object; only a fraction of the projectiles 
in this size range will reach the surface to produce 
a crater. However, detonation either at the sur- 
face or in the lower atmosphere is capable of 
severe local damage, generally on a greater scale 
than that associated with a large nuclear weapon. 
Both the Tunguska (1908) and Meteor Crater 
impacts are small examples of this class. The 
average interval between such impacts for the 
whole Earth is a few centuries; between impacts 
in the inhabited parts of the planet is a few 
millennia; and between impacts in densely 
populated or urban areas is of the order of 1 00, 000 
years. About 300,000 Earth-crossing objects 
probably exist in this size range, with several 
passing between Earth and Moon each year. 

The Spaceguard Survey will discover hundreds of 
objects in this size range every month. By the end 
of the initial 25-year survey, it will be possible to 
track the orbits of as many as 30,000, or about 10 
percent of the total population. If the survey 
continues for a century, the total will rise to about 
40 percent. Since the interval between such 
impacts is greater than 1 00 years, it is moderately 
likely that a continuous survey will detect the 
"next Tunguska" event with ample warning for 



corrective action. However, in contrast to the 
larger ECAs and even the intermediate- and long- 
period comets, this survey will not achieve a near- 
complete survey of Earth-crossing objects in the 
100-m size range in less than several centuries 
with current technology. If there is a societal 
interest in protecting against impacts of this size, 
presumably alternate technologies will be devel- 
oped to deal with them. 

9.2 SURVEY NETWORK: 
COST AND SCHEDULE 

The proposed Spaceguard Survey network con- 
sists of six telescopes of 2- to 3-m aperture together 
with a central clearinghouse for coordination of the 
observing programs and computation of orbits. It also 
requires access to observing time on existing plan- 
etary radars and optical telescopes for follow-up. For 
purposes of this discussion, we assume that the 
Spaceguard Survey will be international in operations 
and funding, with the United States taking a lead- 
ership role through the Solar System Exploration 
Division of NASA's Office of Space Science and Ap- 
plications. 

9.2.1 The Spaceguard Survey Telescopes 

The six survey telescopes required for the 
Spaceguard Survey are new instruments optimized 
for the discovery of faint asteroids and comets. While 
it is possible that one or more existing telescopes could 
be retrofitted for this purpose, we expect that the most 
cost-effective approach is to design and construct 
telescopes specifically for this proj ect. For purposes of 
this Report, we consider a nominal telescope design of 
2.5-m aperture and 5.2-m focal length with a refrac- 
tive prime-focus corrector providing a field-of-view of 
at least 2 deg. The telescope will have altitude- 
azimuth mounting and be capable of pointing to an 
accuracy of a few arcsec and tracking to a precision of 
a fraction of an arcsec at rates up to 20 times sidereal. 
We assume that each telescope will be located at an 
existing observatory site of proven quality, so that no 
site surveys or new infrastructure development (roads, 
power, etc.) is required. The nominal aperture of 
2.5 m is optimized for the ECA survey, but we note 
that larger telescope aperture (3 m or even more) 
would permit long-period comets to be detected at 
greater distances and thereby provide both greater 
completeness and months of additional warning. 

An instrument of very similar design has recently 
been proposed by Princeton University for a wide- 
angle supernova survey. We believe that the 
Spaceguard Survey Telescopes could similarly be built 
for about $6 million each, including observatory 
building, but not including the focal plane of several 



50 • CHAPTER 9 • THE SPACEGUARD SURVEY: SUMMARY 



mosaicked CCD detectors or the supporting data 
processing and computation capability. For each 
telescope, we allocate $1 million for the focal plane 
and $1 million for computer hardware and software, 
for a total cost per installation of $8 million. If these 
six telescopes were purchased together, the capital 
costs would thus be about $48 million. 

For an estimate of operating costs, we assume 
that each telescope will require the following staff- 
ing: 2 astronomers, 2 administrative support person- 
nel, 3 telescope operators, 1 each senior electronic 
and software engineers, and 2 maintenance and sup- 
port technicians, for a total of 11 persons. Additional 
funds will be needed for transportation, power, sleeping 
accommodations for observers, and other routine costs 
associated with the operation of an observatory; the 
exact nature of these expenses depends on the location 
and management of the pre-existing site where the 
telescope is located. The total operations for each site 
should therefore run between $1 .5 million and $2.0 
million per year. In making this estimate we assume 
that each survey telescope is dedicated to the 
Spaceguard effort, and that it will be in use for about 
three weeks (1 00 to 1 50 hours) of actual observing per 
month. If it is intended that the telescope be used for 
other unrelated purposes when the Moon is bright, we 
assume that the other users will pay their pro rata 
share of operation costs. 

The Spaceguard Survey Operations Center should 
provide overall coordination of the international ob- 
serving effort, including rapid communications among 
the survey telescopes and those involved in follow-up 
observations. The Spaceguard Survey Operations 
Center will also compute orbits and ephemerides and 
provide an on-going evaluation of the hazard posed by 
any object discovered by the Survey. Similar func- 
tions are performed today for the much smaller number 
of known asteroids by the Minor Planet Center in 
Cambridge, Massachusetts. Scaling from that opera- 
tion, we estimate an initial cost of $2 million for 
computers and related equipment, and an annual 
operating cost of $2 million. 

A third component of the Spaceguard Survey 
Program is follow-up, including radar and optical 
observations. As noted previously in this Report, it 
would be desirable to have one or more dedicated 
planetary radars and large-aperture optical telescopes 
(4-m class). However, we anticipate that a great deal 
of useful work could be done initially using existing 
planetary radars and optical facilities. Therefore, for 
purposes of this Report, we simply allocate a sum of $2 
million per year for the support of radar and optical 
observing on these instruments. 



9.2.2 Spaceguard Management and Cost- 
Sharing 

The total estimated capital costs for the 
Spaceguard Survey are $50 million, with operating 
costs of $10 to $15 million per year. We anticipate 
that these costs would be shared among several na- 
tions with advanced technical capability, with the 
maximum expenditure for the U.S. (or any other 
nation) of less than half the total amount. For purposes 
of U.S. budgeting, we assume that NASA will pay the 
cost of two telescopes ($1 6 million) and the Operations 
Center ($2 million), and will support operating costs 
of $5 million per year. 

Management of the U.S. component of the 
Spaceguard Survey could be accomplished by NASA 
in one of two ways. (1) The telescopes could be 
constructed and operated by universities or other 
organizations with funding from NASA Headquar- 
ters through grants or contracts, as is done today with 
the NASA IRTF telescope on Mauna Kea (owned by 
NASA but managed by the University of Hawaii 
under a five-year contract) or the 0.9-m Spacewatch 
Telescope on Kitt Peak (owned and operated by the 
University of Arizona with grant support from NASA). 
(2) NASA could construct and operate the telescopes 
itself through one of its Centers (JPL or Ames, for 
example); the Centers might contract with universi- 
ties or industry for operations but would retain a more 
direct management control. Similarly, the Spaceguard 
Survey Operations Center could be located at a NASA 
Center or could be supported by grants or contracts at 
a university or similar location, such as the present 
Minor Planet Center at the Harvard-Smithsonian 
Center for Astrophysics. In any case, international 
cooperation and coordination is essential, and an 
international focus is required from the beginning in 
planning and supporting this program. 

9.2.3 Initial Steps 

The construction of the new Spaceguard Survey 
telescopes will require approximately four years from 
the time funding is available. In the meantime, 
several steps are essential to ensure a smooth transi- 
tion from the present small surveys to the new pro- 
gram. (1) An international coordination effort should 
be initiated by NASA, independent of but coordinated 
with the International Astronomical Union Working 
Group on Near Earth Objects, in order to plan for the 
orderly development of the Spaceguard Survey net- 
work. (2) The small cadre of current asteroid observ- 
ers should be strengthened. Additional expenditures 
of about $1 million per year on existing teams would 
allow for expansion of personnel, purchase of badly 
needed new equipment, and greater sky coverage. 
Consequently, the discovery rate of ECAs should 
more than double, thereby also increasing our confi- 



CHAPTER9 • THE SPACEGUARD SURVEY: SUMMARY • 51 



dence in modeling the population of such objects and 
planning the requirements for operation of the full-up 
survey. (3) In order to gain additional experience 
with the kind of automated CCD scanning techniques 
proposed for the Spaceguard Survey, efforts should be 
made as soon as possible to place in operation a 
telescope that utilizes these techniques; one such 
option is the proposed 1.8-m Spacewatch telescope at 
the University of Arizona. Efforts are also required in 
studying the use of CCD arrays and in developing 
appropriate software to support CCD scanning. (4) 
Continuing support should be provided for research 
on near-Earth asteroids and comets, including their 
dynamics and their physical properties. For purposes 
of this study, we assume an increase of $2 million/year 
beyond current NASA expenditures for these pro- 
grams, to be maintained during the transition period. 

9.2.4 Proposed Schedule for NASA Funding 

On the assumption that the Spaceguard Program 
can begin in a modest way in FY93 and will reach full 
funding about FY95, we suggest the following pos- 
sible schedule for new NASA support of this effort. 

Table 9-1. Proposed NASA Funding (in FY93 $M). 



Fiscal Year 93 94 95 96 97 98 99 



00 



Transition 02 02 02 02 02 02 01 00 

Capital costs 01 02 04 04 04 03 00 00 
Operations 00 00 00 01 02 03 05 05 



Total 



03 04 06 07 08 08 06 



05 



9.3 CONCLUSIONS 

The Spaceguard Survey has been optimized for 
the discovery and tracking of the larger ECAs, which 
constitute the greater part of the cosmic impact haz- 



ard. If any large ECAs threaten impact with the 
Earth, they almost certainly could be discovered with 
ample lead-time to take corrective action. The 
Spaceguard system also will discover most large in- 
coming intermediate- and long-period comets, but the 
warning time may be only a few months. Finally, the 
great majority of the new objects discovered by the 
Spaceguard Survey will have diameters of less than 
1 km; these should be picked up at a rate of about 500 
per month. It is therefore reasonably likely that even 
the "next Tunguska" projectile (20 megatons energy) 
will be found by the Spaceguard Survey if it is continued 
for several centuries. 

The Spaceguard Survey should be supported and 
operated on an international basis, with contributions 
from many nations. The total costs for this system are 
of the order of $50 million in capital equipment, 
prim arily for the six survey telescopes , and $1 to $1 5 
million per year in continuing operating support. 
However, these estimates will vary depending on the 
aperture and detailed design of each telescope, the 
nature of the international distribution of effort, and 
the management of the survey. In particular, larger 
telescopes would be appropriate if greater emphasis 
is to be given to the search for long period comets. 
Whatever the exact cost, however, the proposed sys- 
tem can provide, within one decade of its initial 
operation, a reduction in the risk due to unforeseen 
impacts of about 50 percent at a relatively modest 
cost. Of course, additional and much greater expen- 
diture would be required to deflect an incoming object 
if one should be discovered on an impact trajectory 
with the Earth, but in that unlikely event the cost and 
effort would surely be worth it. The first and essential 
step is that addressed by the Spaceguard Survey: to 
carry out a comprehensive survey of near-Earth space 
in order to assess the population of near-Earth as- 
teroids and comets and to identify any potentially 
hazardous objects. 



52 • CHAPTER 9 • THE SPACEGUARD SURVEY: SUMMARY 



APPENDIX A. 
ASTEROID TABLES 



Table Al . Earth-crossing Asteroids. For each object whose orbit can evolve to intersect that of the Earth, 
the following information is given: the absolute magnitude (H), the approximate diameter, the depth interior 
to the Earth's orbit to which the asteroid can evolve, the orbital perihelion distance (q), semi-major axis (a), 
eccentricity (e), and inclination (i). The inclination is referred to the invariable plane of the solar system and 
each of these orbital elements represents a mean value at the time of Earth orbital crossing. P s is the estimated 
probability of collision with the Earth (number per billion years) using the equations of Shoemaker et al. (1979) 
while P o is the same probability calculated from the equations of E. J. Opik (1951). V. gives the approximate 
impact speed for an Earth collision. For some objects, it is not possible to compute mean orbital elements at 
the time of crossing and for these objects, osculating elements (given in parentheses) have been used as rough 
approximations. Where two sets of elements are given, there are two different conditions of crossing. For those 
objects whose motions are commensurate with Jupiter, no orbital elements are given because they have chaotic 
(unpredictable) motions over the long term. Those objects whose orbits are not sufficiently secure are probably 
lost and may be found only by a serendipitous search. These objects have an (L) following their name. This table 
is based upon the work of E. M. Shoemaker and is current through May 1991. 



ATEN ASTEROIDS 



Object 




H 


Approx. 
Diam 
(km) 


Depth 
(AU) 


q 

(AU) 


a 

(AU) 


e 


i 

(deg) 


P s 
1 

10 9 yr 


P 
1 

10 9 yr 


v, 


Provisional 
Designation No. 


Name 


(km/s) 



1989 VA 17.00 1 0.292 0.729 (0.60) (29) — (4) (22) 

1978RA 2100 Ra-Shalom 16.05 2.4 0.388 0.445 0.832 0.465 13.1 6.3 6.7 17.9 

1954 XA(L) 18.9 0.5 0.203 0.475 0.777 0.389 5.04 34 30 14.5 

1984 QA 3362 Khufu 18.10 0.7 0.559 0.481 0.990 0.514 8.37 5.3 6.2 19.8 

1976UA 2340 Hathor 20.26 (0.2) 0.356 0.486 0.844 0.424 6.27 14 14 16.3 

1986 TO 3753 14.4 3 0.499 0.998 (0.50) (22) (3) (22) 

1991 JY 16.50 2 0.204 0.572 ' 0.940 0.391 44.3 2.9 2.4 26.7 

1989 UQ 19.0 0.5 0.240 0.645 0.915 0.295 1.95 42 43 14.1 

1990VA 19.5 0.4 0.346 0.662 0.985 0.328 13.7 5.4 5.7 16.5 

1986 EB 3554 Amun 15.82 2.0 0.299 0.730 0.974 0.251 21.6 5.4 5.0 17.4 

1976 AA 2062 Aten 16.80 0.9 0.223 0.743 0.966 0.231 18.0 7.1 6.5 16.0 



APPENDIX A • ASTEROID TABLES • A-l 



APOLLO ASTEROIDS 



Object 




H 


Approx 
Diam 
(km) 


Depth 
(AU) 


q 

(AU) 


(AU) 


e 


(deg) 


1 
10 9 yr 


P„ 
1 

10 9 yr 


v, 


Provisional 
Designation No. 


Name 


(km/s) 



1983 TB 3200 Phaethon 14.60 

1949 MA 1566 Icarus 16.40 

1978 SB 2212 Hephaistos 13.87 

1990 UO(L) 20.5 

1990 SM 16.5 

1974 MA (L) 14.0 



5025 P-L(L) 

1984 KB 
1986WA 
1991 AM 
1991 AQ 

1947 XC 

1 936 CA 
1971 UA 
1971 FA 
1982TA 
1 990 BG 

1985 PA 
1987SY 
1979 XB(L) 

1989 PB 

1990 MU 

1991 CB1(L) 
1987 KF 

1986 PA 

1937 UB(L) 

1989 QF 

1987 0A 
1932 HA 

1988 EG 

1990 TG1 

1989 UR 
1989 FC 
1959 LM 

1991 GO(L) 
1991 BA(L) 
1977 HB 

1989 DA 
1973 EA 

1 983 VA 

1990 HA 
1990 UA 
1 989 FB 

1 948 OA 



3838 Epona 



2201 
2101 
1865 
1864 
4197 

3752 
4450 

4769 
4953 

4341 
4034 



Oljato 
Adonis 
Cerberus 
Daedalus 



Camillo 
Pan 



Castalia 



Poseidon 



1862 Apollo 



4581 
4183 



Asclepius 
Cuno 



2063 Bacchus 
1981 Midas 



15.9 

15.5 

15.4 

16.5 

17.5 

15.25 

18.70 

16.84 

14.85 

14.5 

14.0 

15.5 

17.1 

19.0 

16.9 

14.3 

18.0 

15.6 

18.1 

17.0 

18.0 

18.5 

16.25 

19.0 

15.0 

18.0 

20.5 

14.5 

19.0 

28.5 

16.4 

18.0 

15.0 



16.5 

17.0 

19.50.4 

4544 Xanthus 7.1 

1685 Toro 14.23 



6.9 
0.9 
5 
0.2 

1 
5 

2 

1.4 
3 
1 
1 

1.4 
1 

1.0 
(3.1) 
1.8 
5 
2 
1 

0.5 
1.5 
3 
1 
3 
1 
1 
1 
1 

1.4 
1 
3 
1 

0.2 
4 

0.5 
0.006 
1 
1 
1 



0.844 
0.929 
0.771 
0.727 



0.760 



0.613 



0.657 
0.620 
0.504 
0.491 



0.409 



0.522 
(0.56) 
0.478 
0.487 
0.484 
0.512 
0.438 
0.459 
0.418 
0.414 
0.423 
0.399 

0.362 
0.371 
0.425 
0.380 

0.330 
0.602 



2 0.485 

1 0.599 

0.373 0.750 

1 0.250 

5.2 



0.140 
0.205 
0.240 
0.276 
0.395 
0.973 

0.420 
0.429 
0.452 
0.454 
0.500 
0.511 
0.512 
0.526 
0.526 
0.529 
0.544 
0.551 
0.555 
0.566 
0.568 
0.569 
0.580 
0.593 
0.606 
0.624 
0.639 
0.641 
0.647 
0.664 

0.675 
0.679 
0.699 
0.706 
0.713 
0.719 
0.728 
0.735 

0.737 
0.747 
1.721 
0.761 
0.769 



1.271 
1.078 
2.163 
1.234 
2.157 
1.757 

(4.2) 
2.221 
1.505 
1.695 
2.159 
2.174 
1.875 
1.080 
1.461 
2.300 
1.486 
1.414 
1.442 
2.264 
1.063 
1.622 
1.686 
1.836 
1.060 
1.639 
1.155 
1.490 
1.471 
1.270 



(0.89) 
0.810 
0.889 
0.776 
0.817 
0.446 
0.772 

(0.90) 
0.807 

(0.70) 
0.732 

(0.77) 
0.765 
0.727 
0.513 
0.640 

(0.77) 
0.634 

(0.61) 
0.615 

(0.75) 
0.466 
0.649 
0.656 
0.677 
0.428 
0.619 
0.447 
0.570 
0.560 
0.477 



(22) 
18.0 
10.0 
24.1 
11.3 
53.4 
32.8 
(6.2) 
3.37 
(29) 
19.7 
(3.2) 
1.33 
2.03 
14.9 
15.9 
(12) 
26.3 
(32) 

1.85 
(10) 

9.68 
26.4 
9.36 
5.87 
10.0 
5.64 
5.27 
11.0 
6.13 
2.71 



3:1 commensurability 



1.080 
1.023 
1.981 
1.930 
2.161 
1.078 
2.166 
1.776 

2.615 
2.567 
0.564 
1.042 
1.368 



0.375 
0.336 
0.647 
0.634 
(0.67) 
0.333 
0.664 
0.586 
0.652 
0.718 
0.709 
0.81 
0.270 
0.438 



11.5 
4.41 
7.42 
9.55 

(2.2) 
8.99 
5.08 
5.5 

41.3 
6.81 
3.94 
8.3 

13.3 
9.15 



1.8 
0.44 
0.9 
0.4 
0.9 



1.1 

0.5 

2.3 
2.8 
2.5 

1.0 

0.6 

6.4 
(0.5) 
4.2 
0.6 
1.2 
1.5 
4.5 
2.2 
6.4 
1.6 
2.8 
8.8 

4.4 
13.3 
1.3 
1.1 

6.5 
1.5 
3.8 

0.7 
1.1 
12.9 
6.6 
(4) 



(1.4) 
2.2 
1.2 
1.4 
1.1 
2.5 

M) 

3.2 

(1) 
1.0 

(4) 
6.1 
6.2 
3.1 
1.6 

(1) 
1.1 

(1) 
10.5 
(1.2) 
5.1 
0.9 
2.1 
2.9 
5.3 
3.7 
8.1 
2.4 
4.3 
12 

5.1 
15.1 
2.2 
1.9 
(5) 
7.2 
2.8 
0.7 

1.6 

2.7 

18.8 

6.4 

(4.2) 



(35) 
30.6 
34.6 
30.5 
30.0 
32.0 

(32) 
28.8 

(29) 
28.0 

(27) 
26.4 
25.4 
20.9 
26.0 

(27) 
26.3 

(27) 
22.2 

(25) 
18.9 
26.5 
23.4 
23.3 
18.0 
21.7 
17.9 
21.1 
20.3 
18.3 

17.1 
15.4 
21.3 
21.2 
(21) 
15.8 
20.8 
30.7 

21.6 
21.1 

15.5 
17.2 



A-2 • APPENDIX A • ASTEROID TABLES 



APOLLO ASTEROIDS (Cont'd.) 



Object 




H 


Approx 
Diam 
(km) 


Depth 
(AU) 


q 

(AU) 


a 
(AU) 


e 


i 
(deg) 


P s 
1 

10 9 yr 


P 
1 

10 9 yr 


v, 


Provisional 
Designation No. 


Name 


(km/s) 



1988 XB 
1950 DA(L) 
1 978 CA 
1988TA 
1990 UQ 
1988 VP4 
1973 NA(L) 



- — 17.5 

- 15.8 

- 17.8 

- 21.0 

- 17.5 

- 15.8 

- — 15.5 

1990 UN (L) 23.5 

1951 RA 1620 Geographos 15.60 



1987 SB 
1977 HA 
6344 P-L(L) 

1982 HR 

1990 SP 
6743 P-L 

1991 EE 
1991 BB 
1991 DG 

1990 OS 

1991 BN(L) 
1990SS 
1975YA 

1 989 JA 
1986JK 

1983 LC(L) 
1991 JW 
1991 CS 

1989 AZ 
1987QA 

1990 MF 
1948 EA 
1979VA 

1981 VA 

1982 DB 
1982 BB 
1989 UP 
1989 AC 
1976WA 

1991 JX 

1984 KD 
1989VB 



4486 Mithra 
2135 Aristaeus 



3361 Orpheus 



5011 



2102 Tantalus 



4257 Ubasti 



1863 Antinous 

4015 

3360 

4660 Nereus 
3103 



4179 Toutatis 
2329 Orthos 



15.4 

17.94 

21.9 

19.03 

17.0 

17.0 

17.5 

16.0 

18.5 

20.0 

20.0 

19.0 

15.3 

16.5 

19.0 

19.0 

19.5 

17.5 

19.5 

15.8 

18.7 

15.54 

15.99 

16.20 



3671 Dionysus 



18.5 
16.3 
20.0 



1 

2 

1.9 

0.2 

1 

3 

3 

0.06 

2 

3 

1 

0.1 

0.8 

1 

1 

1 

2 

0.6 

0.3 

0.3 

0.6 

2 

2 

1 

0.5 

0.5 

1 

0.4 

3 

0.6 

1.8 

5 

1.8 



18.30 1 

15.38 1.5 

20.7 0.2 

14.0 5 

14.9 3 



0.6 

1 

0.3 



0.263 
0.266 
0.293 
0.262 
0.262 
0.328 



0.363 
0.210 
0.595 



0.207 



0.224 



0.175 
0.251 
0.168 



0.158 



0.334 
0.104 



0.138 



0.163 
0.076 
0.074 
0.346 

0.092 
0.054 



0.295 



0.779 
0.783 
0.784 
0.789 
0.789 
0.790 
0.803 

0.808 
0.808 
0.813 
0.816 
0.822 
0.822 
0.827 
0.837 
0.838 
0.866 
0.870 
0.878 
0.885 
0.886 
0.888 

0.893 

0.904 
0.913 
0.932 
0.935 

0.949 
0.951 
0.955 
0.958 

0.969 
0.974 
0.988 

0.990 



1.011 
1.026 



1.467 0.469 3.98 

1.683 0.535 10.7 

1.125 0.303 25.5 

1.541 0.488 3.42 

1.551 0.491 3.76 

2.263 0.651 10.3 

2.447 0.672 67.9 

0.891 51.8 

1.709 0.527 3.42 

1.245 0.351 14.2 

2.202 0.631 1.06 

1 .600 (0.49) (23) 

2.619 0.686 3.71 

1.209 (0.32) (2.7) 

1.355 (0.39) (13) 

1.635 0.488 6.88 

2.266 (0.63) (10) 

1.186 (0.27) (39) 

1.427 0.390 11.0 

1.672 0.475 0.86 

1.443 0.387 7.58 

1.703 (0.48) (19) 

1.290 0.312 62.5 

0.744 49.1 

1.769 0.495 14.8 
5:2 commensurability 

2.629 0.656 1 .54 

1.038 0.120 8.34 

1.123 (0.17) (37) 

1.649 0.433 9.92 



1.747 0.457 3.02 

2.260 0.579 23.1 
2.645 0.639 5.03 

2.462 0.611 38.6 

0.751 20.8 
1 .490 0.350 4.87 

1.407 0.308 22.0 

1.864 (0.47) (3.9) 
3:1 commensurability 
2.404 0.588 32.6 

0.702 16.9 
3:1 commensurability 

2.198 (0.54) (14) 

1.865 (0.45) (1.6) 



6.2 
1.9 
3.6 
6.3 
5.8 
1.0 
0.5 

5.3 
3.8 
5.5 
(2.0) 



3.8 

4.4 
17.2 
6.4 

2.5 

2.8 

5.8 
30.0 

5.9 

16.6 

0.7 



7.9 
2.4 
3.0 
8.2 
7.5 
1.5 
0.4 

7.0 

3.9 

8.8 
(1.5) 

1.6 
(21) 
(4) 

4.5 
(2) 
(3) 

4.4 
19.3 

6.5 
(2) 

1.5 

2.3 

7.6 
28.9 

(5) 
5.0 

15.2 
1.3 
0.84 
1.6 



17.0 
18.7 
19.5 
17.1 
17.2 
20.1 
40.5 

17.4 
16.7 
18.5 

(21) 
19.1 

(14) 

(17) 
16.7 

(19) 

(24) 
15.8 
15.5 
15.0 

(19) 
34.8 

17.5 

16.8 
12.6 
(22) 

15.1 

14.1 
19.9 
15.5 
26.6 



22.5 


20.3 


13.1 




3.9 


17.3 




(13) 


(14) 


1.8 


2.2 


23.3 




(1.5) 


(16) 




(13) 


(13) 



APPENDIX A • ASTEROID TABLES • A-3 



EARTH-CROSSING AMOR ASTEROIDS 



Object 




H 


Approx 
Diam 
(km) 


Depth 
(AU) 


q 

(AU) 


(AU) 


e 


(deg) 


't 

10 9 yr 


't 

10 9 yr 


v, 

(km/s) 


Provisional 
Designation No. 


Name 



1982 DV 
1980WF 
1988SM 
1918 DB 
1978 DA 
1991 FA 
1991 FB 
1991 FE 

1980 PA 
1987WC 
1991 DB 
1950KA 

1981 QB 
1932 EA1 

1990 VB 
1960 UA 
1968AA 
1982XB 
1929SH 
1973 EC 

1983 RD 
1953 EA 
1987SF3 

1991 JR(L) 
1981 ET3 
1987QB 
1985WA 
1972 RA 
1980AA 
1986 LA 
1986 DA 
1986 RA 



3288 Seleucus 
4688 



887 Alinda 
2608 Seneca 



3908 



1580 Betulia 



4596 



1221 Amor 



2061 Anza 
1917 Cuyo 
3757 



1627 Ivar 
1943 Anteros 
3551 



15.0 

18.6 

18.0 

13.76 

17.52 

17.5 

18.5 

14.9 

17.4 

19.5 

18.5 

14.52 

16.0 

17.7 

16.0 

16.56 

13.9 

18.95 

13.20 

15.75 

16.75 



3122 



1915 Quetzalcoatl 18.97 

19.0 

22.5 

14.2 
19.0 
19.0 
16.8 
19.5 
18.3 
16.0 
16.0 



2202 Pele 



3988 



2.8 

0.6 

1 

4.2 

0.9 

1 

0.5 

4 

1 

0.4 

0.6 

7.4 

2 
1 
2 

(2.7) 
3 

0.5 
8.1 
1.8 
0.9 
0.3 
0.5 
0.1 
4 

0.5 
0.5 
2 

(0.5) 
1 

2.3 
4 



0.319 
0.299 



0.262 
0.218 
0.116 
0.090 



0.079 
(0.27) 

0.064 
0.061 
0.058 
0.050 
0.051 
0.046 
0.042 
0.041 



0.039 
0.038 
0.036 
0.021 



0.018 
0.009 
-0.002 



0.709 
0.730 



0.766 
0.810 
0.912 
0.938 

0.949 
0.957 

0.963 
0.966 
0.970 
0.978 
0.978 
0.981 
0.986 
0.987 



0.989 
0.991 
0.992 
1.006 

1.010 
1.020 
1.026 



2.032 
2.231 



0.651 
0.673 



5.46 
5.38 



3:1 commensurability 
3:1 commensurability 
2.164 0.646 4.51 
2.356 0.656 10.0 
2.451 0.628 3.54 
1.926 0.513 2.80 



1.720 
2.196 

2.240 
1.921 
2.444 
2.263 
2.149 
1.838 
1.863 
1.431 



0.448 
0.564 
0.792 
0.570 
0.497 
0.603 
0.568 
0.545 
0.466 
0.471 
0.310 



14.2 
48.5 
26.4 
28.5 
12.0 
18.5 
3.41 
19.3 
2.55 
7.98 
9.86 



3:1 commensurability 
2.252 0.561 3.56 
1.407 0.296 10.3 
1.768 0.439 20.0 
2.795 0.640 2.34 
5:2 commensurability 
2.291 0.559 8.70 
1.892 0.461 3.55 
1.545 0.336 13.4 
5:2 commensurability 
2:1 commensurability 



0.50 



1.4 
1.1 



1.7 
0.84 
1.6 
4.0 

3.1 

1.7 

0.75 

1.5 

0.34 

1.3 

1.3 

5.1 

1.8 

3.5 



1.1 
5.7 
2.1 
0.26 

0.10 
0.30 



21.0 
20.9 



19.8 
19.7 
16.4 
14.7 

16.0 
30.6 

21.8 
15.4 
18.1 
14.2 
17.8 
13.4 
14.0 
13.4 



13.9 
13.4 
17.0 
14.7 

14.8 
13.2 



A-4 • APPENDIX A • ASTEROID TABLES 



Table A2. Asteroids discovered between May and December 
1991 whose orbits can evolve to intersect that of the Earth. 
Osculating orbital elements are given. 



ATEN ASTEROID 



Provisional 
Designation 


H 


Approx. 
Diam. 
(km) 


q 

(AU) 


e 


(AU) 


1991 VE 


19.5 


0.4 


0.337 


0.617 


0.880 



APOLLO ASTEROIDS 



Provisional 
Designation 


H 


Approx. 
Diam. 
(km) 


q 

(AU) 


e 


(AU) 


1991 RC 


17.0 


1 


0.185 


0.829 


1.082 


1991 LH 


17.0 


1 


0.364 


0.731 


1.352 


1991 TB2(L?) 


17.0 


1 


0.394 


0.836 


2.397 


1991 VL 


14.0 


5 


0.419 


0.771 


1.834 


1991 WA 


17.0 


1 


0.564 


0.643 


1.578 


1991 RB 


19.0 


0.5 


0.749 


0.484 


1.450 


1991 VK 


16.5 


2 


0.911 


0.506 


1.842 


1991 VA(L) 


27.0 


0.01 


0.926 


0.351 


1.426 


1991 TB1 


17.0 


1 


0.942 


0.353 


1.455 


1991 TU(L) 


28.5 


0.005 


0.945 


0.333 


1.416 


1991 TF3(L?) 


19.0 


0.5 


0.957 


0.531 


2.042 


1991 VG 


28.8 


0.001 


0.973 


0.075 


1.051 


1991 VH 


17.0 


1 


0.973 


0.145 


1.138 


1991 XA(L) 


23.5 


0.05 


0.979 


0.571 


2.283 


1991 TT(L) 


26.0 


0.02 


1.002 


0.161 


1.193 



EARTH-CROSSING AMOR ASTEROID 



Provisional 
Designation 


H 


Approx. 
Diam. 
(km) 


q 

(AU) 


e 


(AU) 


1991 OA 


18.0 


1 


1.036 


0.587 


2.508 



APPENDIX A • ASTEROID TABLES • A-5 



Table A3. Short period comets whose periods (P) are less than 20 years and whose perihelion 
distances (q) are less than 1 .1 AU. Because their motions are often chaotic, it is difficult to predict 
whether the orbit of a periodic comet can evolve to intersect the Earth's orbit. However, the following 
comets either could have made, or can make, close Earth approaches. The listed osculating orbital 
elements are appropriate for the latest observed perihelion passages (T). The orbital eccentricity (e) 
is followed by the argument of perihelion (Peri), the longitude of the ascending node (Node) and the 
inclination (Incl). The angular elements are referred to the ecliptic plane and the B1950.0 equinox. 



Comet 


T 


P 


q 


e 


Peri 


Node 


Incl 


Machholz 


1991.55 


5.24 


0.126 


0.958 


14.5 


93.8 


60.1 


Encke 


1990.82 


3.28 


0.331 


0.850 


186.2 


334.0 


11.9 


Helfenzrieder 


1766.32 


4.35 


0.406 


0.848 


178.7 


75.6 


7.9 


Honda-Mrkos-Pajdusakova 


1990.70 


5.30 


0.541 


0.822 


325.7 


88.7 


4.2 


Brorsen 


1879.24 


5.46 


0.590 


0.810 


14.9 


102.3 


29.4 


Lexell 


1770.62 


5.60 


0.674 


0.786 


224.9 


133.9 


1.6 


Denning-Fujikawa 


1978.75 


9.01 


0.779 


0.820 


334.0 


41.0 


8.7 


Biela 


1852.73 


6.62 


0.861 


0.756 


223.2 


247.3 


12.6 


Blanpain 


1819.89 


5.10 


0.892 


0.699 


350.2 


79.2 


9.1 


Schwassmann-Wachmann 3 


1990.38 


5.35 


0.936 


0.694 


198.8 


69.3 


11.4 


Hartley 2 


1991.70 


6.26 


0.953 


0.719 


174.9 


226.1 


9.3 


Grigg-Skjellerup 


1987.46 


5.10 


0.993 


0.665 


359.3 


212.6 


21.1 


Turtle 


1980.95 


13.7 


1.015 


0.823 


206.9 


269.9 


54.5 


Giacobini-Zinner 


1992.28 


6.61 


1.034 


0.706 


172.5 


194.7 


31.8 


Tuttle-Giacobini-Kresak 


1990.11 


5.46 


1.068 


0.656 


61.6 


140.9 


9.2 


Wirtanen 


1991.72 


5.50 


1.083 


0.652 


356.1 


81.6 


11.7 


Finlay 


1988.43 


6.95 


1.094 


0.699 


322.2 


41.7 


3.6 



A-6 • APPENDIX A • ASTEROID TABLES 



Table A4. Close Earth approaches by asteroids and comets (< 0.2 AU) during the 
interval January 1 992 to December 2000. This list includes only those objects known 
to have secure orbits in December 1991. 





Object 


Date 

of Close 

Approach 

(mm/dd/yy) 


Minimum 

Separation 

(AU) 


Minimum 
Separation 

(Lunar 
Distance) 


Approx. 

Diameter 

(km) 


Provisional 
Designation 


No. 


Name 


1973 EA 


1981 


Midas 


3/11/92 


0.134 


52.4 


1 


1 989 AC 


4179 


Toutatis 


12/08/92 


0.024 


9.4 


5 


1989 PB 


4769 


Castalia 


4/08/93 


0.132 


51.6 


1.5 


1982 HR 


3361 


Orpheus 


3/02/94 


0.150 


58.7 


0. 


1990 MU 


4953 




5/30/94 


0.142 


55.6 


3 


1951 RA 


1620 


Geographos 


8/25/94 


0.033 


12.9 




1978 RA 


2100 


Ra-Shalom 


10/12/94 


0.155 


60.6 


2. 


1976AA 


2062 


Aten 


1/12/95 


0.127 


49.7 


0.9 


1976 UA 


2340 


Hathor 


1/16/95 


0.137 


53.6 


0.2 






P/Honda-Mrkos- 












Pajdusakova 


2/04/96 


0.170 


66.5 




1991 JX 






6/08/95 


0.034 


13.3 


0.4 


1977 HB 


2063 


Bacchus 


3/31/96 


0.068 


26.6 


1 


1 949 MA 


1566 


Icarus 


6/11/96 


0.101 


39.5 


0.9 


1982 BB 


3103 





8/06/96 


0.115 


45.0 


1.5 


1982TA 


4197 





10/25/96 


0.085 


33.3 


1.8 


1980 PA 


3908 




1 0/27/96 


0.061 


23.9 


1 


1989 AC 


4179 


Toutatis 


11/29/96 


0.035 


13.7 


5 






P/Encke 


7/04/97 


0.190 


74.3 





1984 KD 


3671 


Dionysus 


7/06/97 


0.114 


44.6 


1 


1978 RA 


2100 


Ra-Shalom 


9/26/97 


0.171 


66.9 


2.4 


1975YA 


2102 


Tantalus 


12/21/97 


0.138 


54.0 


2 


1982 HR 


3361 


Orpheus 


2/12/98 


0.167 


65.3 


0.8 


1988 EG 







2/28/98 


0.032 


12.5 


1 


1971 UA 


1865 


Cerberus 


11/24/98 


0.163 


63.8 


1.0 


1948 EA 


1863 


Antinous 


4/01/99 


0.190 


74.3 


1.8 


1991 JX 






6/02/99 


0.050 


19.6 


0.6 


1986 JK 






7/11/00 


0.122 


47.7 


1 


1987 SB 


4486 


Mithra 


8/14/00 


0.047 


18.4 


3 


1978 RA 


2100 


Ra-Shalom 


9/06/00 


0.190 


74.3 


2.4 


1976 UA 


2340 


Hathor 


1 0/25/00 


0.197 


77.1 


0.2 


1989 AC 


4179 


Toutatis 


10/31/00 


0.074 


29.0 


5 


1959 LM 


4183 


Cuno 


1 2/22/00 


0.143 


56.0 


4 



Source: D. Yeomans 



APPENDIX A • ASTEROID TABLES • A-7 



APPENDIX B 
GLOSSARY 



absolute magnitude 
(of asteroid) 

Amor asteroid 



aperture (telescope) 

aphelion 
Apollo asteroid 

arcminute 
arcsecond 
asteroid 



astrometry 

astronomical unit 

(AU) 

Aten asteroid 



the magnitude an asteroid would have at zero phase angle and at 1 AU from the Earth 
and Sun. 

asteroid having perihelion distance between 1.017 and 1.3 AU. Amor asteroids do not 
cross the Earth's orbit at the present time. 

the diameter of the primary lens or mirror of a telescope; hence, the best single measure 
of the light-gathering power of a telescope. 

the point in an elliptical orbit of a planet, asteroid, or comet that is farthest from the Sun. 

asteroid having semimajor axis greater than or equal to 1.0 AU and perihelion distance 
less than or equal to 1.017 AU. Apollo asteroids cross the Earth's orbit at the present 
time. 

minute of arc, equal to 1/60 degree. 

second of arc, equal to 1/3600 degree. 

an object orbiting the Sun that is smaller than a major planet (sub- km to about 1 ,000 km 
diameter), but shows no evidence of an atmosphere or other types of activity associated 
with comets. Most asteroids are located in a belt between Mars and Jupiter from 2.2 to 
3.3 AU from the Sun. 

precision measurement of position and/or velocity for an astronomical object. 

average distance between the Earth and the Sun, equal to about 150 million km. 



asteroid having semimajor axis less than 1.0 AU and aphelion distance greater than 
0.983 AU. 



C 

CCD 

comet 



charge-coupled device. A solid-state detector used for low-light-level imaging. 

a volatile-rich body that develops a transient atmosphere as it orbits the Sun. The orbit 
is usually highly elliptical or even parabolic (average perihelion distance less than 1 AU; 
average aphelion distance, roughly 10 4 AU). When a comet comes near the Sun, some 
of its material vaporizes, forming a large head of tenuous gas, and often a tail. 



D 

declination 

Doppler effect 



angular distance north or south of the celestial equator. 

apparent change in frequency or wavelength of the radiation from a source due to its 
relative motion in the line of sight. 



APPENDIX B • GLOSSARY • B-l 



E 

ECA 



ECC 



Earth-crossing asteroid. An asteroid whose orbit crosses the Earth's orbit or will at some 
time cross the Earth's orbit as it evolves under the influence of perturbations from 
Jupiter and the other planets. 

Earth-crossing comet. A comet whose period is greater than 20 years and perihelion 
distance is less than 1.017 AU. 



eccentricity (of ellipse) the measure of the degree to which an ellipse is not circular; ratio of the distance between 

the foci to the major axis. 



ecliptic 
ephemeris 



the apparent annual path of the Sun on the celestial sphere. 

(pi. , ephemerides) a list of computed positions occupied by a celestial body over successive 
intervals of time. 



inclination (of orbit) the angle between the orbital plane of a comet or asteroid and the ecliptic plane. 

intermediate-period comet with a period of 20 to 200 years. 
comet 



K 

kiloton 

Kirkwood gaps 



energy equivalent to 1,000 tons of TNT (4.3 x 10 12 Joules). 

regions in the asteroid zone which have been swept clear of asteroids by the perturbing 
effects of Jupiter. They were named for the American astronomer Daniel Kirkwood, who 
first noted them in 1866. 



long-period comet comet with a period greater than 200 years. 



M 

magnitude 
(astronomical) 



main-belt asteroids 

megaton 
meteor 

meteorite 



a number, measured on a logarithmic scale, used to indicate the brightness of an object. 
Two stars differing by 5 magnitudes differ in brightness by a factor of 100. The brighter 
the star, the lower the numerical value of the magnitude; very bright objects have 
negative magnitudes. The star Vega (alpha Lyrae) is defined to be magnitude zero. 

asteroids that occupy the main asteroid belt between Mars and Jupiter, sometimes 
limited specifically to the most populous parts of the belt, from 2.2 to 3.3 AU from the Sun. 

energy equivalent of one million tons of TNT (4.3 x 10 16 Joules). 

the light phenomenon produced by an object experiencing frictional heating when 
entering a planetary atmosphere; also used for the glowing meteoroid itself. If 
particularly large, it is described as a fireball. 

a natural object of extraterrestrial origin that survives passage through the atmosphere. 



B-2 • APPENDIX B • GLOSSARY 



N 
NEO 

new comet 



near-Earth object. Objects whose orbits bring them near the Earth. Specifically Apollo, 
Amor, and Aten Asteroids, and certain comets. 

comet on its first approach to the Sun. 



O 

Oort comet cloud 



opposition 



a spherical cloud of comets having semimajor axes greater than 20,000 AU. Comets in 
this shell can be sufficiently perturbed by passing stars or giant molecular clouds so that 
a fraction of them acquire orbits that take them within the orbits of Jupiter and Saturn. 

an angle of 180° between a planet, the Earth, and the Sun. 



P 

perihelion 

perturbation 
phase angle 
power law 



the place in the orbit of an object revolving around the Sun where it is closest to the Sun. 

for a body orbiting the Sun or a planet, the gravitational effect of a third body (e.g., 
another planet) on its orbit, usually resulting in small changes or periodic fluctuations. 

the solar phase angle: the angle subtended at the center of a planet or other body by the 
directions to the Sun and the observer. 

a mathematical relation in which the resulting value is dependent upon a variable being 
raised to an exponential power. 



R 

resonance 

right ascension 



an orbital condition in which one object is subject to periodic gravitational perturbations 
by another, most commonly arising when two objects orbiting a third have periods of 
revolution that are simple multiples or fractions of each other. 

a coordinate for measuring the east-west positions of celestial bodies; the angle mea- 
sured eastward along the celestial equator from the vernal equinox to the hour circle 
passing through a body. 



S 

Schmidt telescope 



semimajor axis 
(of orbit) 

short-period comet 

sidereal period 

steradian 



a type of reflecting telescope (more accurately, a large camera) in which the coma 
produced by a spherical concave mirror is compensated for by a thin correcting lens 
placed at the opening of the telescope tube. The Palomar 1 .2-m Schmidt has a usable field 
of 6°. 

half the major axis of an ellipse. For a planetary orbit, it represents the body's average 
distance from the Sun. 

comet with a period less than 20 years. 

the time it takes for a planet or satellite to make one complete revolution relative to the 
stars. 

the solid angle which, having its vertex in the center of a sphere, cuts out an area of the 
surface of the sphere equal to that of a square with sides of length equal to the radius of 
the sphere. A sphere contains 4k steradians. 



APPENDIX B • GLOSSARY • B-3 



APPENDIX C 
REFERENCES 



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American Institute of Aeronautics and Astronautics, 
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Baldwin, R. B., 1949. The Face of the Moon. Univer- 
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Bowell, E., K. Muinonen, andE. M. Shoemaker, (1991). 
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Earth Asteroids Conference, San Juan Capistrano, 
June 1991. 

Chapman, C. R. and D. Morrison, 1989a. Cosmic 
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Chapman, C. R. and D. Morrison, 1989b. American 
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Clarke, A. C, 1973. Rendezvous with Rama. Ballantine 
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Covey, C, S. J. Ghan, J. J. Walton, and P. R. 
Weissman, 1990. Global environmental effects of 
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Everhart, E . , 1 967. Intrinsic distributions of cometary 
perihelia and magnitudes. Astron. J. 72:1 002-1 01 1 . 

Fernandez, J. A. and W. -H. Ip, 1991. Statistical and 
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in the Post-Halley Era (R. L. Newburn, Jr., M. 
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Gehrels, T., 1991. Scanning with charge-coupled de- 
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Grieve, R. A. F., 1991. Terrestrial impact: the record 
in the rocks. Meteoritics 26:175-194. 

Helin, E. F. and E. M. Shoemaker, 1977. Discovery of 
Asteroid 1976 AA. Icarus 31:415-419. 



Helin, E. F. and R. S. Dunbar, 1984. International 
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Helin, E. F., N. D. Hulkower, and D. F. Bender, 1984. 
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Helin, E. F., andR. S. Dunbar, 1990. Search Techniques 
for Near-Earth Asteroids. Vistas in Astronomy 
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Muinonen, K. and E. Bowell, 1992. Asteroid orbital 
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Institute, Houston, in press. 

Muinonen, K., E. Bowell, E. M. Shoemaker, and R. F. 
Wolfe, 1991. Discovery of Earth-crossing aster- 
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Paper presented at the Near-Earth Asteroids Con- 
ference, San Juan Capistrano, June 1991. 

Olsson-Steel, D., 1987. Collisions in the Solar System 
- IV. Cometary impacts upon the planets. Mon. 
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Opik, E. J., 1951. Proceedings of the Royal Irish Acad- 
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Ostro, S. J., D. B. Cambell, J. F. Chandler, I. I. 
Shapiro, and A. A. Hine, 1991. Asteroid radar 
astrometry. Astron. J. 102:1490-1502. 

Pike, J., 1991. The Sky Is Falling: The Hazard of 
Near-Earth Asteroids. The Planetary Report. 
11:16-19. 

Rabinowitz, D. L., 1991 . Detection of Earth-approach- 
ing asteroids in near real time. Astron. J. 101:1518- 
1529. 

Sharpton, V. I. and P. D. Ward, eds, 1990. Global 
Catastrophes in Earth History; An Interdiscipli- 
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Mortality. Special paper/The Geological Society of 
America, Inc., Boulder, CO: 247. Conference held 
October 20-23, 1988, Snowbird, UT. 



Helin, E. F., and E. M. Shoemaker, 1979. Palomar 
Planet-Crossing Asteroid Survey 1973-1978. 
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APPENDIX C • REFERENCES • C-l 



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C-2 • APPENDIX C • REFERENCES 



NASA 

National Aeronautics and 
Space Administration